how do I reinstall firefox 3.6.3?

You can get the installer for 3.6.3 from ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/3.6.3/win32/en-US/ Are you choosing 3.6.3 because it is the last version before the handling of plugins was introduced? If you are and you want to revert to the pre-3.6.4 handling of plugins, therre is no need to install 3.6.3. Instead follow the instructions for disabling crash protection at http://kb.mozillazine.org/Plugin-container_and_out-of-process_plugins It is best not to use old versions of Firefox as they will have published security holes.