Use 1.3.4 for a period, it worked fine.
But later version 1.3.5~1.4.1 all cease working, the addon won't modifiy YT page and can't remember my setting...
Is this due to settings saved in Sync? If this really the cause I would have a feature request to have a option "Save Setting Locally"
(not everyone using Sync, for security reason)
Firefox 52.9.0 32-bit without any Addon
Thanks a lot ^^