Is this memory that is needed for some other application you are running? Is your system running low on virtual memory? If not, what is the problem with FF using it? From what I've seen (at least with FF6), when system memory gets low FF will start to release memory back.
I'd rather have FF use the memory for cache to speed up performance than to have it sit unused.
I'd rather have FF use the memory for cache to speed up performance than to have it sit unused.