I have a 2011 MBP with 2.4GHz i7 cpu and 4GB ram, and Mac OS X 10.7.4. I like to keep several Firefox windows open, and I've noticed if I reboot the MBP and launch Firefox (which is set to reopen all the previous windows), then the system runs fine initially. But then after a few days of having Firefox open, the system starts slowing down, and I frequently see that spinning rainbow circle thing which means the system is struggling to do whatever it's trying to do and the fan starts running at maximum loudness. I didn't think it was Firefox, since I don't see this problem on Windows. (I have the latest Firefox 13.0.1 installed on Mac OS X.) But now I have closed all the Firefox windows while leaving Firefox still running, and the Activity Monitor is showing a "firefox" process taking up 100.8% of the CPU and 12KB of memory, and a different "Firefox" process taking up 760MB of RAM and 0.0% of the CPU. So maybe it is Firefox that's slowing down the system. Oh, and now I told Firefox to quit, and there's no longer the dot under the Firefox icon in the dockbar which means it isn't running, but Activity Monitor still shows the "firefox" process and a couple of Firefox plugin processes. Time to reboot. I'm going to try running Chrome for a while to see if I get the same slowdown.