Deal with Tor Launcher being integrated into Tor Browser 9.0 and Firefox 68esr possibly removing support for XUL/XPCOM extensions

Originally created by @intrigeri on #15709 (Redmine)

For Tor Browser 9.0, based on Firefox 68esr, the plan is to integrate Tor Launcher into Tor Browser, while still allowing us to disable it at runtime, and to run it as a XUL/XPCOM extension. It’s not 100% clear to me if this implies we can still run it as a XUL app as we’ve been doing so far. This work “is planned for 9.0a1 which likely will ship in April”.

And once Tor Browser is ported to Firefox 68esr, we probably won’t be able to run it as a XUL app, since XUL/XPCOM extensions won’t be supported by Firefox anymore. See the dedicated ticket: #16448 (closed).

https://trac.torproject.org/projects/tor/wiki/org/meetings/2018MexicoCity/Notes/TBBMeetingDays explains a bit the challenges & goals.

Upstream tickets:

• https://trac.torproject.org/projects/tor/ticket/28044
• https://trac.torproject.org/projects/tor/ticket/30473

Feature Branch: feature/16356-tor-browser-9.0+force-all-tests

Related issues

• Related to #14555 (closed)
• Related to #16448 (closed)
• Related to #15144 (closed)
• Related to #17215 (closed)