why would you remove firstparty.isolate? What if it remains enabled? Does something break?

Mmm… I don’t think so… firstparty.isolate do a lot of things. Actually with temporary container correctly configured you can think about turn it off, but IMHO it’s better to follow arkenfox user.js configurations. Actually, you can make your question on his github https://github.com/arkenfox/user.js/issues