I started having this problem recently myself. I tried all the solutions, repairing permissions, repairing disk, rebuilding disk, removing plist, upgrading to 10. Nothing has helped. Someone mentioned space on the hard drive. I'm about 80% full right now. Could that be a problem?

Attaching strace to firefox's PID I see thousands of munmap and madvise function calls going on while firefox hangs, completely at random, for minutes at a time. These hangs occur while I am scrolling a page, typing in the location bar, while a page is loading, and sometimes even when there's no interaction with the browser at all.

Ermm, I actually have this problem but after reading quite a bit of the others statement, I tweaked with the advanced settings of ff. I dunno but i feel like its kinda smoother now. I ticked off the use hardware acceleration when available and then at the network tab I ticked on the override automatic cache management and set it to 0. I got the idea off of a vegas tut on the lag part. I dunno hope it might help. Im still keeping chrome just in case.