Occasionally I'm getting the message that add-on installation has been blocked even on https://addons.mozilla.org, and apparently this happens to others as well (https://www.reddit.com/r/firefox/comments/7tcx3w/help_blocked_addon/). Resetting "xpinstall.whitelist.add" and restarting Firefox fixes this, but it might eventually happen again and of course it shouldn't even have ever occurred in the first place.

Thanks to a helpful comment from that Reddit thread, I've realised that this happens when Site Settings are cleared. The problem is that after clearing site settings our default permissions (xpinstall.whitelist.add) aren't restored. A quick fix would be to add some code to PermissionsUtils.jsm that a) resets the corresponding prefs themselves [1] b) and the list of prefs that have already been imported [2] and call that when clearing Site Settings through our Sanitizer [3]. A more holistic approach would be to finally tackle bug 1072744 so we're back to using the same system as Desktop. [1] https://dxr.mozilla.org/mozilla-central/rev/e22e2c9eb81686e958a9448ea3d1e8cd766743e2/toolkit/modules/PermissionsUtils.jsm#89-92 [2] PermissionsUtils.jsm [3] https://dxr.mozilla.org/mozilla-central/rev/e22e2c9eb81686e958a9448ea3d1e8cd766743e2/mobile/android/modules/Sanitizer.jsm#124-125

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