- I'm likely to return back to some of them. I might not know which ones. Typing in the address bar brings them back fast and the page does not need to be loaded again. Having the tab already open is also a strong signal that this is what I'm looking up.
- no noticeable slowdown anyway, Firefox is actually quite efficient.
- I don't care for taking the time of closing them progressively. It happens that I will close them all at the same time at some point when I feel like I need some clean up. Usually when I'm done with something.
- I think I learned to mostly ignore this part of the screen. Everything happens in the address bar.
In short, it's a combination of intentionally leaving tabs open so I can go back to them later without reloading the page, and not wanting to spend the time to manage them.
I usually have under 100 tabs open though, often even fewer.
With a good UI the unused ones just don't bother you anymore anyway until you scroll or filter them. They show me my train of thoughts without having to consciously organize anything. Unused tabs get unloaded from RAM anyway, so the cost of keeping them open is minimal.
A few years ago there was a version of Firefox that didn't slow down and opened quickly even at tens of thousands of tabs, but unfortunately it quickly regressed, so throwing everything out periodically is still inevitable:)