> You can search across tabs using the tab search in the top right corner though (an arrow pointing down).

Which uses the same broken stuff, so you can't actually search. Check this ticket: https://bugzilla.mozilla.org/show_bug.cgi?id=1479858

Btw, the arrow icon cannot be moved from the tabs bar, annoying because I completely hide this bar, but sometimes it would be useful to have this menu.

I also have this hidden and using tree style, however I don't see a point. What is useful about this menu?

I don't open 400 at once, they accumulate over time (I mostly open about 50-200 at once). And of course I am using tree style tabs, otherwise I would not be able to use more than 50 tabs efficiently (right now I have 71 opened, because I cleaned it up at the end of june). But that is not the point.

The arrow tab search does the exact same thing as using % modifier in the address bar. I still can't search pages that are in containers that are different from the currently opened site.

Oh well I wasn't aware this limitation applied there too (don't use it too often myself). I agree this is bad behavior.