Firebug is most likely the culprit [1]. Firefox 15 will have a general fix for the kind of memory leaks such as the one caused by Firebug (affectionately called "Hueyfix" after its inventor Kyle Huey by some) [2].

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=669730 [2] https://bugzilla.mozilla.org/show_bug.cgi?id=695480

Yeah, Firebug is one of those leaky addons. Give 15 (Aurora) a shot; other Firebug users have said that it's much improved.