It is very likely that one single site is responsible for this. For example in my usage, Jenkins portal is leaking memory like hell, will fill up a few GBs in a a day and jack up CPU utilization to 100%.

I has hundreds of tabs at that point, so I had to find out the culprit. What I did was open a task manager and expand Fifefox process to see dozens of subprocesses. I then sorted by memory (or by cpu) to find out worst offender and killed only that single subprocess without touching others. And after doing that I've looked at the list of tabs and saw that one of them has changed to the crash report tab, with a visibly different icon. And looking at it I saw that it was originally Jenkins portal. Now I proactively close it's tabs and leaks stopped. Maybe this will help you.