I have Firefox 63.0.1 and beginning with 63 I started noticing high cpu usage coming from Firefox, that for certain sites, e.g., Facebook, Messenger, Wunderground (all which were very slow before). Eventually it was using 100% cpu for all sites, so much so none would load, very frustrating. I use Kali Linux and its one account is root, and this is the account where this was happening. I even tried Firefox ESR, which is 60 and same problem. So, I created another user so I could use Chromium to look for a fix. Then I tried the exact same Firefox application on this new account, works better than ever. I am perplexed as to why this situation would be and how to fix it. All I can think of is there is some process running behind the scenes that is conflicting with some process on the root account, but wouldn't know where to begin to look. My version of 63 is an updated 60, if that helps any. This si so bad that when I try to use Firefox it stops everything else from working because it is using all the resources. I also noticed that memory usage has gone way up, to where Firefox and its processes use almost 100% of memory, this occurs over time whether I am using sites or it is just sitting. The same happens in safe mode, so it doesn't appear to be extensions, although my system monitor says web extensions in both safe mode and regular mode.