I've been having this problem - but just did check to validate fonts in Fontbook. No bad fonts as such, but several fonts were flagged 'minor problem proceed with caution'. After removing these, font cache cleaned, running 'all maintenance tasks' in Ice Clean and system rebooted this problems seems to have been resolved. (for me at least).

If anyone else having this issue finds resolving font management issues does or doesn't fix this, please post so can help pin down cause of this problem.

Open Activity Monitor (AM) and watch CPU activity. Sort processes list by % CPU utilization. FF should come to the top. Open a web site that gives you a long page. Resize FF window so you can see AM at the same time and watch FF % while you scroll rapidly up and down the page. You can see that this sometimes pegs the CPU to 100%. When that happens you'll get a spinning beachball. The question is - why does it take so much CPU cycles to scroll a window in FF? I'm on version 3.6.17 and get the beachball almost every time I scroll. Do the same experiment with Safari and note the difference - Safari % hardly changes at all no matter how fast you scroll. Go figure.