Seems to show you've still got a third or just under available. Do you run any add-Ons or plug-ins with Firefox?
Also worth noting that Firefox is only comparatively efficient when under a heavy load but can be the least efficient of the main browser types when only displaying a low number of tabs.
There are a couple of graphs at the following link showing comparisons (admittedly running Windows 7 so not sure how relevant it is!)
http://www.tomshardware.com/reviews/...k,3108-13.html