I think I have the same problem. Whenever FF is opened my entire system starts to lag which is especially noticable in my mouse jumping all over the place whenever I move it.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window. You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")

I've got a similar problem. I have only noticed it when I have many tabs active at once. I have 8 GB ram and a high-end gaming computer.

It stopped after I disabled Shockwave Flash. I have not tested it thoroughly but it seems like a quick fix for now.

I know it's a year old question and all... But still.

I have "only" had this problem since the middle of december 2011. So it came out of nowhere. I then upgraded to v3.6.25 from v3.6.24 but to no avail it just keeps locking up for a second with 10 second intervals approx. and it gets worse the longer FF is opened. I have only upgraded a few of my add-ons such as Gmail Watcher and Downloadhelper. Maybe they are the culprits but I haven't tried to disable them yet. Even now that I'm writing this FF keeps freezing. Annoying. Anyone else have this problem and use the 2 before mentioned add ons?

I've had this since Windows 7 updated on Sunday the 22nd of April.

After 20 minutes FF is unusable. It takes over 5 seconds to switch open tabs. Opening a new tab takes up to 30 seconds. Any page which requires text entering lags by a good 5 seconds for every key press. Scrolling is totally random, sometimes a line, some time a screen and, again, lagging, move the scroll wheel and sometime later the page might move... Hovering over a button, nothing is detected for, again, a good 5 seconds and even then press any on screen button lags for many seconds before anything happens.

If I restart FF the problem clears for 20 minutes and then is back.

I've tried disabling add ons and extensions, no changes, I've updated to FF v12, no change. Its almost as if some buffer somewhere isn't clearing ??

In any event FF is this state is painful to use and I'm being forced to switch to something else, which , after I've spent months tweaking FF the way I want it, is flustering to say the least. 

Both IE and Chrome work without a problem, the issue is only with FF.

My system has 8 gig of ram which should be enough for anything to run smoothly. . .