https://bugs.freedesktop.org/show_bug.cgi?id=78366
--- Comment #3 from Alex Deucher agd5f@yahoo.com --- (In reply to comment #2)
Apparently, memory is recovered by closing Firefox. Top doesn't show it using that much memory, but it somehow does in less traceable ways.
Server side resources: http://www.freedesktop.org/wiki/Software/xrestop/