Just from the usability and performance characteristics alone, I'd rather retire Chromium than Firefox.
Chromium used to perform visibly better than Firefox with somewhere around one to five tabs open, so sometimes, if I didn't have the browser running and didn't want its full session to load, I've started Chromium just to quickly check one URL - however, recent versions of Firefox made it comparable in such cases, so the only use case for Chromium I have now is to deal with badly coded websites that assume it's the only browser out there.
When you tell me you open thousands of tabs, I wonder if you’re just stress testing the browser. I expect software not to work well when I stress test it.
My common browser usage is to have low hundreds of tabs open - right now it's a bit higher than usual, 1086, although it's not the record. It's just regular browsing, topic research, some social media and stuff left for later. Works well - under Firefox with Tab Center Redux, that is. I've tried switching to Chromium in the past, when Firefox used to be visibly slower, but I've never could use it for longer, always went back to Firefox as Chromium's UX was horrible. With recent versions, I don't even feel the need to check out Chromium ever again.
I'm currently using Chrome on a 4GB system with 10 tabs open with no performance issues or fans screaming. To claim that Chrome is borderline unusable on 8GB of RAM is a lie.
Or maybe people have different standards/expectations?