Domain isolation toggle
Some websites, particularly file hosts, expect a user's IP not to change. If they are spread over multiple domains, the domain isolator causes them to refuse to work, possibly with a message such as "Wrong IP".
FWIW, commenting line 2018 in torbutton.js will conveniently allow the pref extensions.torbutton.use_nontor_proxy to toggle isolation, which fixes all site issues.
Can't find that pref toggled anywhere else. Torbutton doesn't even appear to support nontor proxies.
This is split off from #16409 (moved).