I tried all common troubleshooting steps: deleting cache reinstalling different settings turning off adblocker/vpn disabled efficiency in task manager and set it permanently to off in FF I used to have 1000+ tabs in 2016 no issue. I had problem in past with Firefox using 100% CPU which now it is not using only like 50, but memory is getting full 89% i use also large page file on ssd. Problem is even when i restart firefox it is slow with low RAM usage! SO that's not it! Even 5 tabs sometimes it is slow! It is so damn slow it drives me crazy! I tried also Quantum in past when Firefox has 100% CPU usage bug and it was only tiny bit better but still caused 100% CPU usage! All other browsers are anti user cannot even stop Autoplay literal terrorist Firefox is only normal browser unfortunately performance compare to 2016 got absolutely abysmal! I had this problem even in-between reinstals of OS so it doesn't seem it is me. I play demanding games no issues on my PC...