Ok, and I'm sure you did uninstall reboot reinstall. Shit, I can't think of what that can be. Any themes installed? I know that can count as extensions but in case you overlooked it I'm mentioning it. I haven't seen this. Aside from a few things that I mentioned I can't think of what would be causing it maybe try to redownload a fresh copy. Idk I'm in it I'm just trying to help.
No themes. And I did try all kinds of stuff. One thing for sure. Waterfox doesn't do that for me so I've been using that. Maybe it was a windows issue at the time. (Maybe to push people to use edge. But that's too much of a conspiracy theory.) Waterfox works without that issue on windows and Linux for me and my friend who also switched after the same issue.
I appreciate you trying to help.
They don't recognize it officially but people talk about all the time. It seems it's because Firefox doesn't have a tab freezing feature built in for inactive tabs so websites might have JavaScript errors that can cause this. There is literally a post on Firefox subreddit from 2 days ago with people talking about it. A friend of mine also recently finally caved and switched from Firefox to opera because his ram gradually got used more and more the longer he had Firefox open.
Good luck with this argument. This instance of firefox was there 3,000 years ago. I use it for work. When its up it gets to maybe twice that. A gig when videos are on tabs.
All I can say is what my experience was and what the experience of people I know was. I don't know what else to tell you other than that and that there are other people noticing similar things.