Tor Launcher: deal with Firefox 68esr removing support for XUL/XPCOM extensions

Following up on #15709 (closed), once Tor Browser is ported to Firefox 68esr, we probably won’t be able to run Tor Launcher as a XUL app, since XUL/XPCOM extensions won’t be supported by Firefox anymore.

So we’ll probably need to replace Tor Launcher with something else entirely.

Related issues

Related to #15709 (closed)
Related to #15144

Original created by @intrigeri on 16448 (Redmine)

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