A user.js file is reported in your profile folder. Check to see what software added it or the changes you applied there yourself, and temporarily run Firefox after moving it elsewhere. Is the performance similar?

And of course, check the performance in Firefox’s Safe Mode to see if one of your add-ons is causing the issue.

Note that you may have to reset prefs that are modified/set via user.js via the right-click context menu on the about:config page to the default value.

Tonnes said

A user.js file is reported in your profile folder. Check to see what software added it or the changes you applied there yourself, and temporarily run Firefox after moving it elsewhere. Is the performance similar? And of course, check the performance in Firefox’s Safe Mode to see if one of your add-ons is causing the issue.

moved user.js file and reset firefox, which now seems to be acting more normally - didn't see a way to check what software added it and date was two years ago, so hard to understand why this was suddenly a problem, but happy it's happy now.

cor-el said

Note that you may have to reset prefs that are modified/set via user.js via the right-click context menu on the about:config page to the default value.

thank you, I did this also, and happy to report success. This is an older desktop with only 4GB RAM, so as websites increase ad volume, places bigger burden on system, which may be part of the problem.

Hi there! Anyone w/ the same problem?? After the latest Sep. 2018 update, FF fills the RAM to 3.5 G or more! (out of my 4 G). Only 4 tabs are open, no new thing w/ FF or my computer. Tried all the previous suggestions, no use. PC gets so slow that even mouse might not move! I don't have the user.js but deleted the java profile file and let FF make a new one, still the same! I updated Java and Flash, got a little better, but still something is wrong. Thanks