Beware that privacy.firstparty.isolate might break some sites that depend on third-party services for login authentication or comment systems. If you find any broken sites, you can report them in Bugzilla and mention this Firefox bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=1299996

I use noscript to by default turn off scripts from 3rd party sites (unless whitelisted). It breaks quite a few sites, but it also makes lots of sites load quicker and doesn't load a bunch of advertising/tracking stuff that I don't really want loaded anyway.