I can confirm both that newer firefox is notably more responsive than old, and that I (or an extension, though I don't have many) managed to get a ff profile into a state where it had inexplicably bad performance, esp. unpredictable long freezes.

Bigger problem are the apps that consume too much CPU and memory. Unfortunately there is no nice way to avoid their killing the whole system (Linux here). But other browsers are no better. The browser makers should realize they are delivering a multiuser OS and protect the resources accordingly.

Something really unusual is happening then. I'd second trying with a new profile. You can get general troubleshooting advice at https://support.mozilla.org/en-US/kb/firefox-takes-long-time...

(disclaimer: i work for mozilla, but not on the browser)