try completely removing firefox
I found this on some website
1. Go to Control Panel>Add/Remove Programs in XP or Control Panel>Programs & Features in Vista, and if a 'Mozilla Firefox' entry (or similar) exists, select it and click Change/Remove and uninstall it. Reboot if required.
2. Go to your Documents and Settings\[username]\Application Data\Mozilla directory in XP, or the \Users\[username]\AppData\Local\Mozilla and \Users\[username]\AppData\Roaming\Mozilla\ directories in Vista and delete the entire \Firefox subdirectory and all of its contents. Note that this will delete all of your bookmarks and saved settings, so if you want to backup any or all of your current Profile, see the Customizing Firefox section on page 7 of this guide first. Typically all that I would recommend you backup and restore is your bookmarks.html file.
3. Go to your \Program Files\Mozilla Firefox\ directory and delete it and all of its contents.
4. you can use the Registry Editor (Start>Run>Regedit) and delete the following keys - that is, right click on their name in the left pane of Registry Editor and select Delete:
[HKEY_CLASSES_ROOT\FirefoxHTML]
[HKEY_CURRENT_USER\Software\Classes\Applications\firefox.exe]
[HKEY_CURRENT_USER\Software\Mozilla]
[HKEY_CURRENT_USER\Software\MozillaPlugins]
[HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla]
[HKEY_LOCAL_MACHINE\SOFTWARE\mozilla.org]
[HKEY_LOCAL_MACHINE\SOFTWARE\MozillaPlugins]
then try reinstalling the latest version, and make sure you have to latest flash player