I have the exact same problem described by Davecabezo, but only when I have firefox running for several hours at a time.
For example, when I close FF at the end of the day, the process Firefox.exe starts consuming 100% of my processing power and increasing its memory usage from `90,000k to ~200,000k.
This happens for 10-40 seconds.
I've not some research as to why this happens, but I haven't gotten anywhere.
What I have now, in case this happens, is as batch file with the following command:
taskkill /f /im firefox.exe
Simple, but effective, and is associated with a keyboard shortcut.
Note: When this process is terminated, FF doesn't ask if you want to restore the previous session. It goes about as if nothing has happened.