You say only with Firefox, but you don't say if you have tried another browser: obviously the simplest method to try before troubleshooting either Firefox, Gnome or the desktop/window manager in use..
I use Opera, but there are others such as chromium (or chrome)http://www.bestinternetbrowser.com/review/linux
--you might want to try a smaller web browser, such as Epiphany..http://en.wikipedia.org/wiki/Web_(web_browser
--for a faster web browser, a textstyle such as lync is possible, but people who have gotten used to GUIfied interfaces might find it too sparse, even though it would be very efficient after some practice..http://en.wikipedia.org/wiki/Lynx_(web_browser
I can also note that it may be some Gnome+application problem, for example I installed Gnome commander onto Mint lmde 13 and while it does run, it seems to hang when doing a large file search interfering with the desktop (a stuck open window), if I leave it alone long enough, eventually it responds..
--usually I am too impatient to wait fro that..