Make sure your graphics drivers are up to date. At least on Windows, Firefox is trying to use your graphics card for rendering by default, which it hasn't done for most of its existence (it's a fairly new feature). This of course triggers tons of bugs in graphics drivers that are otherwise not designed to handle the pressures of rendering the web.
Try creating a new Mozilla profile by passing the -P or -profilemanager switches to the command (this is very platform/version/program-specific so I'll leave it to Mozilla to explain in depth): http://kb.mozillazine.org/Profile_Manager