It is said that Firefox may have more than one memory leak, and in any case the mem usage can grow a lot bigger than the memory cache would allow for if you leave it running long enough.
Short answer: This won't fix the memory leaks completely.
Long answer: This won't fix all of the leaks, but it may fix some of them, and this could slow down the RAM-gobbling process. In any case it should not be harmful, so it's worth a try.