GTK (Gimp Tool Kit) is a set of programs that draw the user interface - what you actually see and use. It's a bit of Firefox that's different in Linux to Windows or Mac, which have their own equivalents. The newer version of Firefox needs a newer set of GTK program libraries than you have. You may or may not be able to upgrade GTK - other parts of your system use it. The best way is to look for a version of Firefox 3.6 that has been built specially for your version of Linux. But there may not be one. Are you using Ubuntu? If so look at Ubuntuzilla http://sourceforge.net/apps/mediawiki/ubuntuzilla/index.php?title=Main_Page If not what Linux distro and version are you using?

See also http://en.wikipedia.org/wiki/GTK%2B