Drop the use of resource:// for own modules

https://bugzilla.mozilla.org/show_bug.cgi?id=903959
Fix broken languages button in about:preferences

Nightly 33.0a1
https://bugzilla.mozilla.org/show_bug.cgi?id=754304
Some manifest flags for a more strict chrome registration

It prevents a CSS warning in ancient versions of the application

Meanwhile, collecting dust:
https://bugzilla.mozilla.org/show_bug.cgi?id=380401
Don't touch preferences on quit after being disabled or uninstalled
Saner check for building the toolbar button as a widget; renaming