I found this while reviewing the patch for bug 1390445. STR (mostly same as bug 1390445): - Install the "Language Tool Grammar Checker 1.0.13" add-on (or one of other ones listed in that bug, but I only checked this one). - Ensure that OOP webextensions are enabled (should be the default on Windows) - Run the build so as to get console output (since this is Windows, need to use git bash or cygwin shell) - Go to about:addons, go to the options for the addon, scroll down to the dropdowns, and click on a dropdown. Actual: - Console reports an error about the PAPZCTreeManager::Msg_SetTargetAPZC message. The message was deserialized, but the handler returned false (indicating failure). Expected: - No error I verified that the layers id mismatch guard is getting hit at [1] although I'm not really sure why. This doesn't appear to have a user-facing effect, t least not that I can see. [1] http://searchfox.org/mozilla-central/rev/298033405057ca7aa5099153797467eceeaa08b5/gfx/layers/ipc/APZCTreeManagerParent.cpp#214