- 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.