Power users interested in this niche feature can use a script: https://old.reddit.com/r/firefox/comments/1m594nv/multi_tab_...

I think with UserChromeJS you can run TabMixPlus too. I don't remember if it needs anything else. (Or a fork like Waterfox, of course.)

But like, if we're turning signature checking and sandboxing off, we're getting pretty far from stock Firefox. And of course, they can (and often will) break it on every update.