Bug 1778559 - Just disable addon button if policy forbids extension installation. r=devtools-reviewers,jdescottes

Differential Revision: https://phabricator.services.mozilla.com/D248342

browser/components/enterprisepolicies/Policies.sys.mjs

@@ -1330,8 +1330,7 @@ export var Policies = {
             "extensions.htmlaboutaddons.recommendations.enable",
             false
           );
-          // Block about:debugging
-          blockAboutPage(manager, "about:debugging");
+          manager.disallowFeature("installTemporaryAddon");
         }
         if ("restricted_domains" in extensionSettings["*"]) {
           let restrictedDomains = Services.prefs
@@ -1661,7 +1660,7 @@ export var Policies = {
       if ("Default" in param) {
         setAndLockPref("xpinstall.enabled", param.Default);
         if (!param.Default) {
-          blockAboutPage(manager, "about:debugging");
+          manager.disallowFeature("installTemporaryAddon");
           setAndLockPref(
             "browser.newtabpage.activity-stream.asrouter.userprefs.cfr.addons",
             false

devtools/client/aboutdebugging/src/components/debugtarget/TemporaryExtensionInstaller.js

@@ -42,6 +42,7 @@ class TemporaryExtensionInstaller extends PureComponent {
         {
           className: `${className} default-button qa-temporary-extension-install-button`,
           onClick: () => this.install(),
+          disabled: !Services.policies.isAllowed("installTemporaryAddon"),
         },
         "Load Temporary Add-on…"
       )