diff --git a/extensions/auth/nsAuthGSSAPI.h b/extensions/auth/nsAuthGSSAPI.h
index 5fce23b4ef46..a0430b42f41f 100644
--- a/extensions/auth/nsAuthGSSAPI.h
+++ b/extensions/auth/nsAuthGSSAPI.h
@@ -41,7 +41,7 @@ public:
     NS_DECL_THREADSAFE_ISUPPORTS
     NS_DECL_NSIAUTHMODULE
 
-    nsAuthGSSAPI(pType package);
+    explicit nsAuthGSSAPI(pType package);
 
     static void Shutdown();
 
diff --git a/modules/libjar/nsJAR.h b/modules/libjar/nsJAR.h
index 43b032512052..b7c716de39f0 100644
--- a/modules/libjar/nsJAR.h
+++ b/modules/libjar/nsJAR.h
@@ -140,7 +140,7 @@ public:
     NS_DECL_THREADSAFE_ISUPPORTS
     NS_DECL_NSIZIPENTRY
 
-    nsJARItem(nsZipItem* aZipItem);
+    explicit nsJARItem(nsZipItem* aZipItem);
 
 private:
     virtual ~nsJARItem() {}
@@ -167,7 +167,7 @@ public:
     NS_DECL_THREADSAFE_ISUPPORTS
     NS_DECL_NSIUTF8STRINGENUMERATOR
 
-    nsJAREnumerator(nsZipFind *aFind) : mFind(aFind), mName(nullptr) {
+    explicit nsJAREnumerator(nsZipFind *aFind) : mFind(aFind), mName(nullptr) {
       NS_ASSERTION(mFind, "nsJAREnumerator: Missing zipFind.");
     }
 
diff --git a/modules/libjar/zipwriter/nsDeflateConverter.h b/modules/libjar/zipwriter/nsDeflateConverter.h
index 51c4778708d9..95ee17586cd1 100644
--- a/modules/libjar/zipwriter/nsDeflateConverter.h
+++ b/modules/libjar/zipwriter/nsDeflateConverter.h
@@ -31,7 +31,7 @@ public:
         mLevel = 6;
     }
 
-    nsDeflateConverter(int32_t level)
+    explicit nsDeflateConverter(int32_t level)
     {
         mLevel = level;
     }
diff --git a/modules/libpref/Preferences.cpp b/modules/libpref/Preferences.cpp
index b3f850ac24db..6827afe219c1 100644
--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -105,7 +105,7 @@ public:
   ValueObserverHashKey(const char *aPref, PrefChangedFunc aCallback) :
     mPrefName(aPref), mCallback(aCallback) { }
 
-  ValueObserverHashKey(const ValueObserverHashKey *aOther) :
+  explicit ValueObserverHashKey(const ValueObserverHashKey *aOther) :
     mPrefName(aOther->mPrefName), mCallback(aOther->mCallback)
   { }
 
diff --git a/modules/libpref/nsPrefBranch.h b/modules/libpref/nsPrefBranch.h
index ed629e686309..d93817c565d5 100644
--- a/modules/libpref/nsPrefBranch.h
+++ b/modules/libpref/nsPrefBranch.h
@@ -78,7 +78,7 @@ class PrefCallback : public PLDHashEntryHdr {
     }
 
     // Copy constructor needs to be explicit or the linker complains.
-    PrefCallback(const PrefCallback *&aCopy)
+    explicit PrefCallback(const PrefCallback *&aCopy)
       : mDomain(aCopy->mDomain),
         mBranch(aCopy->mBranch),
         mWeakRef(aCopy->mWeakRef),
diff --git a/profile/dirserviceprovider/nsProfileDirServiceProvider.h b/profile/dirserviceprovider/nsProfileDirServiceProvider.h
index bf00523923a5..3754a896d5a0 100644
--- a/profile/dirserviceprovider/nsProfileDirServiceProvider.h
+++ b/profile/dirserviceprovider/nsProfileDirServiceProvider.h
@@ -71,7 +71,7 @@ public:
   virtual nsresult         Shutdown();
 
 protected:
-                           nsProfileDirServiceProvider(bool aNotifyObservers = true);
+                           explicit nsProfileDirServiceProvider(bool aNotifyObservers = true);
    virtual                 ~nsProfileDirServiceProvider();
 
   nsresult                 Initialize();
diff --git a/security/manager/ssl/tests/unit/tlsserver/lib/TLSServer.cpp b/security/manager/ssl/tests/unit/tlsserver/lib/TLSServer.cpp
index c4f89d47db39..337a36451876 100644
--- a/security/manager/ssl/tests/unit/tlsserver/lib/TLSServer.cpp
+++ b/security/manager/ssl/tests/unit/tlsserver/lib/TLSServer.cpp
@@ -29,7 +29,7 @@ struct Connection
   PRFileDesc *mSocket;
   char mByte;
 
-  Connection(PRFileDesc *aSocket);
+  explicit Connection(PRFileDesc *aSocket);
   ~Connection();
 };
 
diff --git a/toolkit/components/downloads/ApplicationReputation.cpp b/toolkit/components/downloads/ApplicationReputation.cpp
index 931f73c0a421..2d7e7bc59b5b 100644
--- a/toolkit/components/downloads/ApplicationReputation.cpp
+++ b/toolkit/components/downloads/ApplicationReputation.cpp
@@ -212,7 +212,7 @@ public:
   NS_DECL_NSIURLCLASSIFIERCALLBACK
 
   // Constructor and destructor
-  PendingDBLookup(PendingLookup* aPendingLookup);
+  explicit PendingDBLookup(PendingLookup* aPendingLookup);
 
   // Look up the given URI in the safebrowsing DBs, optionally on both the allow
   // list and the blocklist. If there is a match, call
diff --git a/toolkit/components/osfile/NativeOSFileInternals.cpp b/toolkit/components/osfile/NativeOSFileInternals.cpp
index 311209c961a7..a674df14e10d 100644
--- a/toolkit/components/osfile/NativeOSFileInternals.cpp
+++ b/toolkit/components/osfile/NativeOSFileInternals.cpp
@@ -92,10 +92,10 @@ struct ScopedArrayBufferContentsTraits {
 };
 
 struct ScopedArrayBufferContents: public Scoped<ScopedArrayBufferContentsTraits> {
-  ScopedArrayBufferContents(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM):
+  explicit ScopedArrayBufferContents(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM):
     Scoped<ScopedArrayBufferContentsTraits>(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT)
   { }
-  ScopedArrayBufferContents(const ArrayBufferContents& v
+  explicit ScopedArrayBufferContents(const ArrayBufferContents& v
                             MOZ_GUARD_OBJECT_NOTIFIER_PARAM):
     Scoped<ScopedArrayBufferContentsTraits>(v MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT)
   { }
@@ -171,7 +171,7 @@ public:
    * @param aStartDate The instant at which the operation was
    * requested.  Used to collect Telemetry statistics.
    */
-  AbstractResult(TimeStamp aStartDate)
+  explicit AbstractResult(TimeStamp aStartDate)
     : mStartDate(aStartDate)
   {
     MOZ_ASSERT(NS_IsMainThread());
@@ -277,7 +277,7 @@ AbstractResult::GetResult(JSContext *cx, JS::MutableHandleValue aResult)
 class StringResult MOZ_FINAL : public AbstractResult
 {
 public:
-  StringResult(TimeStamp aStartDate)
+  explicit StringResult(TimeStamp aStartDate)
     : AbstractResult(aStartDate)
   {
   }
@@ -328,7 +328,7 @@ StringResult::GetCacheableResult(JSContext* cx, JS::MutableHandleValue aResult)
 class TypedArrayResult MOZ_FINAL : public AbstractResult
 {
 public:
-  TypedArrayResult(TimeStamp aStartDate)
+  explicit TypedArrayResult(TimeStamp aStartDate)
     : AbstractResult(aStartDate)
   {
   }
diff --git a/toolkit/components/startup/nsAppStartup.cpp b/toolkit/components/startup/nsAppStartup.cpp
index dff063e621e1..8ce16ef6a2a4 100644
--- a/toolkit/components/startup/nsAppStartup.cpp
+++ b/toolkit/components/startup/nsAppStartup.cpp
@@ -101,7 +101,7 @@ private:
   nsRefPtr<nsAppStartup> mService;
 
 public:
-  nsAppExitEvent(nsAppStartup *service) : mService(service) {}
+  explicit nsAppExitEvent(nsAppStartup *service) : mService(service) {}
 
   NS_IMETHOD Run() {
     // Tell the appshell to exit
diff --git a/toolkit/components/telemetry/Telemetry.cpp b/toolkit/components/telemetry/Telemetry.cpp
index f1b6f4a5bc09..bea32fab3e94 100644
--- a/toolkit/components/telemetry/Telemetry.cpp
+++ b/toolkit/components/telemetry/Telemetry.cpp
@@ -77,7 +77,7 @@ template<class EntryType>
 class AutoHashtable : public nsTHashtable<EntryType>
 {
 public:
-  AutoHashtable(uint32_t initLength = PL_DHASH_DEFAULT_INITIAL_LENGTH);
+  explicit AutoHashtable(uint32_t initLength = PL_DHASH_DEFAULT_INITIAL_LENGTH);
   typedef bool (*ReflectEntryFunc)(EntryType *entry, JSContext *cx, JS::Handle<JSObject*> obj);
   bool ReflectIntoJS(ReflectEntryFunc entryFunc, JSContext *cx, JS::Handle<JSObject*> obj);
 private:
@@ -306,7 +306,7 @@ class TelemetryIOInterposeObserver : public IOInterposeObserver
   };
 
 public:
-  TelemetryIOInterposeObserver(nsIFile* aXreDir);
+  explicit TelemetryIOInterposeObserver(nsIFile* aXreDir);
 
   /**
    * An implementation of Observe that records statistics of all
diff --git a/toolkit/components/url-classifier/HashStore.h b/toolkit/components/url-classifier/HashStore.h
index ede320bb9ef2..c33323f05c72 100644
--- a/toolkit/components/url-classifier/HashStore.h
+++ b/toolkit/components/url-classifier/HashStore.h
@@ -22,7 +22,7 @@ namespace safebrowsing {
 // new hashes.
 class TableUpdate {
 public:
-  TableUpdate(const nsACString& aTable)
+  explicit TableUpdate(const nsACString& aTable)
       : mTable(aTable), mLocalUpdate(false) {}
   const nsCString& TableName() const { return mTable; }
 
diff --git a/toolkit/components/url-classifier/nsUrlClassifierProxies.h b/toolkit/components/url-classifier/nsUrlClassifierProxies.h
index c8e9fd1a3087..3ab1948a1e5c 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierProxies.h
+++ b/toolkit/components/url-classifier/nsUrlClassifierProxies.h
@@ -21,7 +21,7 @@ class UrlClassifierDBServiceWorkerProxy MOZ_FINAL :
   public nsIUrlClassifierDBServiceWorker
 {
 public:
-  UrlClassifierDBServiceWorkerProxy(nsIUrlClassifierDBServiceWorker* aTarget)
+  explicit UrlClassifierDBServiceWorkerProxy(nsIUrlClassifierDBServiceWorker* aTarget)
     : mTarget(aTarget)
   { }
 
@@ -162,7 +162,7 @@ class UrlClassifierLookupCallbackProxy MOZ_FINAL :
   public nsIUrlClassifierLookupCallback
 {
 public:
-  UrlClassifierLookupCallbackProxy(nsIUrlClassifierLookupCallback* aTarget)
+  explicit UrlClassifierLookupCallbackProxy(nsIUrlClassifierLookupCallback* aTarget)
     : mTarget(new nsMainThreadPtrHolder<nsIUrlClassifierLookupCallback>(aTarget))
   { }
 
@@ -194,7 +194,7 @@ private:
 class UrlClassifierCallbackProxy MOZ_FINAL : public nsIUrlClassifierCallback
 {
 public:
-  UrlClassifierCallbackProxy(nsIUrlClassifierCallback* aTarget)
+  explicit UrlClassifierCallbackProxy(nsIUrlClassifierCallback* aTarget)
     : mTarget(new nsMainThreadPtrHolder<nsIUrlClassifierCallback>(aTarget))
   { }
 
@@ -227,7 +227,7 @@ class UrlClassifierUpdateObserverProxy MOZ_FINAL :
   public nsIUrlClassifierUpdateObserver
 {
 public:
-  UrlClassifierUpdateObserverProxy(nsIUrlClassifierUpdateObserver* aTarget)
+  explicit UrlClassifierUpdateObserverProxy(nsIUrlClassifierUpdateObserver* aTarget)
     : mTarget(new nsMainThreadPtrHolder<nsIUrlClassifierUpdateObserver>(aTarget))
   { }
 
diff --git a/toolkit/crashreporter/nsExceptionHandler.cpp b/toolkit/crashreporter/nsExceptionHandler.cpp
index b15161100479..cabad55ce2d4 100644
--- a/toolkit/crashreporter/nsExceptionHandler.cpp
+++ b/toolkit/crashreporter/nsExceptionHandler.cpp
@@ -295,7 +295,7 @@ static const int kMagicChildCrashReportFd = 4;
 static Mutex* dumpMapLock;
 struct ChildProcessData : public nsUint32HashKey
 {
-  ChildProcessData(KeyTypePointer aKey)
+  explicit ChildProcessData(KeyTypePointer aKey)
     : nsUint32HashKey(aKey)
     , sequence(0)
 #ifdef MOZ_CRASHREPORTER_INJECTOR
@@ -323,7 +323,7 @@ static nsIThread* sInjectorThread;
 class ReportInjectedCrash : public nsRunnable
 {
 public:
-  ReportInjectedCrash(uint32_t pid) : mPID(pid) { }
+  explicit ReportInjectedCrash(uint32_t pid) : mPID(pid) { }
 
   NS_IMETHOD Run();
 
@@ -1710,7 +1710,7 @@ class DelayedNote
   DelayedNote(const nsACString& aKey, const nsACString& aData)
   : mKey(aKey), mData(aData), mType(Annotation) {}
 
-  DelayedNote(const nsACString& aData)
+  explicit DelayedNote(const nsACString& aData)
   : mData(aData), mType(AppNote) {}
 
   void Run()
diff --git a/toolkit/profile/nsToolkitProfileService.cpp b/toolkit/profile/nsToolkitProfileService.cpp
index e1a174d1c32d..05b3ed42ee5d 100644
--- a/toolkit/profile/nsToolkitProfileService.cpp
+++ b/toolkit/profile/nsToolkitProfileService.cpp
@@ -154,7 +154,7 @@ private:
         NS_DECL_ISUPPORTS
         NS_DECL_NSISIMPLEENUMERATOR
 
-        ProfileEnumerator(nsToolkitProfile *first)
+        explicit ProfileEnumerator(nsToolkitProfile *first)
           { mCurrent = first; }
     private:
         ~ProfileEnumerator() { }
diff --git a/toolkit/xre/nsAppRunner.cpp b/toolkit/xre/nsAppRunner.cpp
index 91149535617f..b98e27b634bb 100644
--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -1330,7 +1330,7 @@ public:
   NS_DECL_ISUPPORTS
   NS_DECL_NSIFACTORY
 
-  nsSingletonFactory(nsISupports* aSingleton);
+  explicit nsSingletonFactory(nsISupports* aSingleton);
 
 private:
   ~nsSingletonFactory() { }
diff --git a/xpcom/tests/TestDeadlockDetector.cpp b/xpcom/tests/TestDeadlockDetector.cpp
index a55caff4605e..0fbdd395216e 100644
--- a/xpcom/tests/TestDeadlockDetector.cpp
+++ b/xpcom/tests/TestDeadlockDetector.cpp
@@ -58,7 +58,7 @@ public:
     nsCString mStdout;
     nsCString mStderr;
 
-    Subprocess(const char* aTestName) {
+    explicit Subprocess(const char* aTestName) {
         // set up stdio redirection
         PRFileDesc* readStdin;  PRFileDesc* writeStdin;
         PRFileDesc* readStdout; PRFileDesc* writeStdout;
diff --git a/xpfe/appshell/nsContentTreeOwner.cpp b/xpfe/appshell/nsContentTreeOwner.cpp
index 98f79c395bfc..2bf7c984e871 100644
--- a/xpfe/appshell/nsContentTreeOwner.cpp
+++ b/xpfe/appshell/nsContentTreeOwner.cpp
@@ -58,7 +58,7 @@ using namespace mozilla;
 class nsSiteWindow : public nsIEmbeddingSiteWindow
 {
 public:
-  nsSiteWindow(nsContentTreeOwner *aAggregator);
+  explicit nsSiteWindow(nsContentTreeOwner *aAggregator);
   virtual ~nsSiteWindow();
 
   NS_DECL_ISUPPORTS
diff --git a/xpfe/appshell/nsContentTreeOwner.h b/xpfe/appshell/nsContentTreeOwner.h
index 0fb674237752..cb4d4514aa6b 100644
--- a/xpfe/appshell/nsContentTreeOwner.h
+++ b/xpfe/appshell/nsContentTreeOwner.h
@@ -43,7 +43,7 @@ public:
    NS_DECL_NSIWINDOWPROVIDER
 
 protected:
-   nsContentTreeOwner(bool fPrimary);
+   explicit nsContentTreeOwner(bool fPrimary);
    virtual ~nsContentTreeOwner();
 
    void XULWindow(nsXULWindow* aXULWindow);
diff --git a/xpfe/appshell/nsWebShellWindow.cpp b/xpfe/appshell/nsWebShellWindow.cpp
index a441f2a8964f..297bfcc41e0f 100644
--- a/xpfe/appshell/nsWebShellWindow.cpp
+++ b/xpfe/appshell/nsWebShellWindow.cpp
@@ -444,7 +444,7 @@ namespace mozilla {
 class WebShellWindowTimerCallback MOZ_FINAL : public nsITimerCallback
 {
 public:
-  WebShellWindowTimerCallback(nsWebShellWindow* aWindow)
+  explicit WebShellWindowTimerCallback(nsWebShellWindow* aWindow)
     : mWindow(aWindow)
   {}
 
diff --git a/xpfe/appshell/nsWebShellWindow.h b/xpfe/appshell/nsWebShellWindow.h
index 8d56876a352a..cd703704ba7d 100644
--- a/xpfe/appshell/nsWebShellWindow.h
+++ b/xpfe/appshell/nsWebShellWindow.h
@@ -28,7 +28,7 @@ class nsWebShellWindow MOZ_FINAL : public nsXULWindow,
                                    public nsIWidgetListener
 {
 public:
-  nsWebShellWindow(uint32_t aChromeFlags);
+  explicit nsWebShellWindow(uint32_t aChromeFlags);
 
   // nsISupports interface...
   NS_DECL_ISUPPORTS_INHERITED
diff --git a/xpfe/appshell/nsXULWindow.h b/xpfe/appshell/nsXULWindow.h
index acd17f0245f2..c08530d92781 100644
--- a/xpfe/appshell/nsXULWindow.h
+++ b/xpfe/appshell/nsXULWindow.h
@@ -80,7 +80,7 @@ protected:
      PAD_SIZE =         0x4
    };
 
-   nsXULWindow(uint32_t aChromeFlags);
+   explicit nsXULWindow(uint32_t aChromeFlags);
    virtual ~nsXULWindow();
 
    NS_IMETHOD EnsureChromeTreeOwner();
diff --git a/xpfe/components/directory/nsDirectoryViewer.h b/xpfe/components/directory/nsDirectoryViewer.h
index 957da4b32c09..0bbb37357df2 100644
--- a/xpfe/components/directory/nsDirectoryViewer.h
+++ b/xpfe/components/directory/nsDirectoryViewer.h
@@ -79,7 +79,7 @@ protected:
     nsIInterfaceRequestor*       mRequestor; // WEAK
     nsCOMPtr<nsIRDFResource>     mDirectory;
 
-    nsHTTPIndex(nsIInterfaceRequestor* aRequestor);
+    explicit nsHTTPIndex(nsIInterfaceRequestor* aRequestor);
     nsresult CommonInit(void);
     nsresult Init(nsIURI* aBaseURL);
     void        GetDestination(nsIRDFResource* r, nsXPIDLCString& dest);