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It doesn't matter if I have 50, 100 or 400, the search just doesn't work across containers. However it is extremely annoying if I have a lot of tabs, because tree style tab also has its limits(though much higher than with the normal UI) and once those are reached it would be far more efficient to search.

You can search across tabs using the tab search in the top right corner though (an arrow pointing down). There might be a shortcut for it but I'm not aware of any.

Although if you are really opening 400 tabs at once I'd recommend either tree-style tabs or rethinking your life choices lol.

> 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.