You want to know the worst site. This site as I write this text. the CPUs are jumping from 0 to 100% steadily.
Strange, it uses almost no CPU here, perhaps as I have scripting turned off. I'll turn it on.
OK, that increased CPU usage. A couple of points, some animated gifs can cause a lot of CPU usage, some bug in our port I believe, they show up here if I turn on JavaScript. Typing doesn't seem to make it much worse.
I also notice heavy CPU usage on the main forum page if there are any new comments, this goes away if no new comments. Once again likely GIF related.
We had a similar problem at Mantis, after I tracked it down Lewis changed all the GIFs to PNGs, which fixed it.
Edit, I'm using SeaMonkey, which isn't much different code wise as most code is shared. There is some differences such as Chatzilla, which causes high CPU usage always if enabled, whether actually being used or not. Same XPI is fine on Firefox and it is the same as was in the 38ESR tree.