ditch Flash
It's pretty much all the time usually. I usually have a bunch of tabs open, so maybe some of them have Flash. Safari with the same number though wouldn't use up so much power. Firefox is almost always the top CPU user, and a lot of the time I end up running at 100% solid.
The number one thing that slows down FireFox (and Safari) is Flash.
If you ditch Flash, it can seem like getting a whole new computer.
The downside is that almost all web-based video sites use Flash, but you can easily turn it on and off for that if you want.
As an alternative, try an AdBlocker.
I find that even when Flash isn't slowing down a site, it's seems like it's some crap Java-based advertisement. So if you want to leave Flash and Java enabled, you can let the AdBlocker eliminate all the crap that's most likely slowing down the browser.
Half the time a browser will get the webpage in seconds, but you have to wait a full minute for some stupid script, (Java or Flash) in an advertisement to load because some idiot has coded it that way. I find this happens a lot on the Mac because some advertisement on a tech page has code that tries to check the Windows registry or something without even thinking that someone might not have Windows, or a "registry."
Finally, I find FireFox just slower in general. Safari is far better than it used to be and is noticeably faster that FireFox IMO.