I've been having problems with heavy resource use on firefox, particularly of CPU. After checking individual exenstions I found that Kaspersky browser extension was the culprit and seriously hogs both RAM and CPU. Turning it off solves the resource problem.
As I have full (paid for!!!!) Kaspersky suite installed I'm not sure what the logic is of having to instal another app to manage browsing for the same product.
I think the developers need to look at this issue.