Honest question, how is FireFox becoming unusable for you? I am using it on both Linux and Windows and do not really have any problems with it. Sure, sometimes a web site does not load properly but more often than not is because some add-ons like ad-blocker and no-script are intervening for me. In other cases, its obvious that the website was never tested in anything but chrome. In those cases where I really need it, I have some chromium-based browser as backup on all my devices. But daily work on FireFox is just fine for me.
Your comment reminds me of how people were complaining for a long time that FireFox had become so slow, while I was browsing with multiple dozens of open tabs on my 8GB system all along. I do not want to say that this is not the case for you, but I honestly cannot reproduce it on three different laptops.