This article has an alternate method for uninstalling when the control panel method doesn't work: Uninstalling Firefox | How to | Firefox Help. (See the box below #5)

If you are planning to reinstall, you might want to make a backup of your Firefox bookmarks and settings. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

You may only see orphaned entries in Add and Remove Programs if you can't locate the Firefox folder in the Firefox program folder (C:\Program Files\Mozilla Firefox\)
In that case you need to edit the registry to remove the entries or use a registry cleaner.