Bug 1862450 - Made installer changes for dual browser extension to MacOS r=mhughes

Depends on D192478

Differential Revision: https://phabricator.services.mozilla.com/D192897

browser/components/BrowserContentHandler.sys.mjs

@@ -543,7 +543,15 @@ nsBrowserContentHandler.prototype = {
         "private-window",
         false
       );
-      if (privateWindowParam) {
+      // Check for Firefox private browsing protocol handler here.
+      let privateProtocolFound = false;
+      let urlFlagIdx = cmdLine.findFlag("url", false);
+      if (urlFlagIdx > -1 && cmdLine.length > 1) {
+        privateProtocolFound = cmdLine
+          .getArgument(urlFlagIdx + 1)
+          .startsWith("firefox-private:");
+      }
+      if (privateWindowParam || privateProtocolFound) {
         let forcePrivate = true;
         let resolvedURI;
         if (!lazy.PrivateBrowsingUtils.enabled) {
@@ -551,6 +559,10 @@ nsBrowserContentHandler.prototype = {
           // access to private browsing has been disabled.
           forcePrivate = false;
           resolvedURI = Services.io.newURI("about:privatebrowsing");
+        } else if (privateProtocolFound) {
+          // We can safely remove the flag and parameter now.
+          const privateProtocolURI = cmdLine.handleFlagWithParam("url", false);
+          resolvedURI = resolveURIInternal(cmdLine, privateProtocolURI);
         } else {
           resolvedURI = resolveURIInternal(cmdLine, privateWindowParam);
         }