A few days ago Firefox started becoming unusable for me.
I usually have more than 10 windows and probably 40 tabs open at any time.
It was so slow I switched for a few days to Edge, which was much more responsive.
But I couldn't give up. I've been with Netscape Navigator, Mozilla Firebird and Firefox all my browsing life.
I fould this page from Mozilla really helpful.
In particular the option to "Minimize memory usage" in the about:memory page.
A combination of actions can really have an impact on the usability.