Ah, crazy Firefox. There’s apparently a known bug that causes a slow but sure memory leak with Firefox. The basic solution is:
- Type about:config in the address bar
- Right click the page and add a New->Integer
- Name: enter browser.cache.memory.capacity
- Value: enter 16000
- Close Firefox completely.
- Fire it up again and enjoy 🙂
Discussed at greater length on the mozillazine forums.