I usually have a lot of tabs open in Firefox while I work. I’ve noticed (as have many others) excessive memory consumption by the browser at times. Right now firefox.exe is using 121,484K with only 9 tabs open. I rebooted first thing this morning, and I’ve had Firefox running for only about four hours.
Most of the information I’ve found says the problem has to do with misbehaving Add-ons, extensions or themes. I’m only running four Add-ons, and decided to uninstall all but my del.icio.us Buttons and Google Browser Sync. Unfortunately, the problem persisted even after a reboot.
I did some more searching and found this thread that suggests loading this image to see if your browser memory consumption goes through the roof. Mine did. I read further down the thread and found a problematic add-on is indeed Google Browser Sync.
Check out this list of problematic extensions to see if any of your favorite add-ons are listed. If none of your extensions are listed, try the suggestions found on the Standard Diagnostic for Firefox and the causes for Firefox Hangs.