A common reason for this is extensions which break Firefox's ability to clean up RAM usage. See this bug and its dependencies https://bugzilla.mozilla.org/show_bug.cgi?id=700547 (Firebug, LastPass, and Scriptish/Greasemonkey seem to be commonly at fault.)

If it's not an extension, it could be a "zombie compartment" and any help you can provide tracking that down would be great. https://developer.mozilla.org/en/Zombie_compartments