Fix Tor-Launcher issues for Firefox 75

Apart from legacy/trac#32174 (moved), legacy/trac#33862 (moved) and legacy/trac#33890 (moved), there seems to be some issues that need to be addressed.

By testing, I found that the "Back" button in the wizard does not have a label and the window used for displaying the bridge captcha and errors is not shown.

By looking at the code and console errors I also found that getAnonymousElementByAttribute is used, but this was removed in https://bugzilla.mozilla.org/show_bug.cgi?id=1591145. Besides, it seems now the <wizard> should not be the root element anymore: https://bugzilla.mozilla.org/show_bug.cgi?id=1584283. The same with <dialog> https://bugzilla.mozilla.org/show_bug.cgi?id=1585482.

I did not look in-depth, but these are some issues I found testing a bit after fixing (in principle) legacy/trac#32174 (moved), legacy/trac#33862 (moved) and legacy/trac#33890 (moved).