Earlier quoted context omitted.
I am genuinely interested: why so many and how can you handle 400 open tabs?? How do you navigate between them? I am lost as soon as I have more than 10 tabs open at the same time...
Oh, that's not hard. First, Firefox doesn't squeeze tabs past certain limit, so you don't have these tiny-tiny tabs that you can't click. You get the scrolling, but it won't bother you after a while. Thus, visually even with 1000 or 2000 tabs browser UI stays clean. Second, when you type stuff into the address bar Firefox searches through opened tabs and suggests switching to that. This minimizes the number of times…
This is how my Firefox looks like:
https://dosowisko.net/Screenshot_20171030_203716.png
https://dosowisko.net/Screenshot_20171030_203804.png
Once a while I'm getting to 1000-2000 open tabs and this is a point where the whole UI starts to feel a bit heavy, which is the only reason I clean them up then - navigating is a non-issue.