Renice goes to my head

I find myself using renice more and more. If you don't know, it lets you change the scheduling priority of a process that's currently running, anywhere from -20 (highest) to 20 (lowest). Processes all start at 0. So if firefox is starting to take the entire processor (as it has been known to do), i renice it to 10 or so. And then I laugh as my computer speeds up and firefox cries. I've tried taking the opposite approach and changing it to -10 to see if it stops lagging. But it doesn't, it just makes the rest of my computer lag too. So no more chances, firefox. Maybe if a process pleased me I might bump it up, but I doubt it, I'm too busy punishing firefox. Renice is far too much power, I don't think I can handle it.