Merge lp:~jbicha/firefox/dont-depend-on-gtk2 into lp:~mozillateam/firefox/firefox-beta.artful

Firefox has only one file that naturally depends on gtk2:
/usr/lib/firefox/gtk2/libmozgtk.so

I think that file is only used by the Flash plugin.

See https://bugzilla.mozilla.org/1377445

All the Flash plugin packages in Ubuntu and Debian already depend on libgtk2.0-0
- adobe-flashplugin
- flashplugin-installer (from source flashplugin-nonfree )
- pepperflashplugin-nonfree (works in Chromium not Firefox but mentioned for completeness)

Therefore, I propose we use the same hack we've used in several other packages to drop the depends on gtk2. The final packages in the default 18.04 install to pull in gtk2 are Firefox, Thunderbird, and debconf (for libgtk2-perl). The tracker bug for that issue is LP: #1585903

The only risk I see is that it is possible for someone to manually try to install Flash without using the Ubuntu packages but I don't understand why anyone would do that and I don't think we should worry about that use case.

I haven't tested Thunderbird yet. I believe the change is unsafe on Thunderbird 52, but I think it would be fine for Thunderbird 57 or 58 Beta. Based on the release schedule, I think we could ship 59 (Beta?) with Ubuntu 18.04 LTS.