Yes, there is a memory leak…

If somebody ever said, there is no memory leak in Firefox, here’s the proof:

Firefox memory leak

It can’t be, that a browser with 4 open tabs requires about 600 MB of memory. That’s 150 MB per Web site, which seems a bit too much ;)

Safari, by the way, requires 128 MB for 5 open tabs.

If you find that Firefox’s memory usage continues to grow after long periods of being open, you may want to consider periodically restarting Firefox to bring the memory usage back to reasonable levels.

http://kb.mozillazine.org/Memory_Leak