Below are all files/folders associated with Firefox. Be aware that deleting these files/folders removes all profiles, preference settings and extensions.
/Applications/Firefox.app (the application)
/Users/<username>/Library/Preferences/org.mozilla.firefox.plist (the preferences)
/Users/<username>/Library/Application Support/Firefox (your profiles folder)
/Users/<username>/Library/Application Support/Mozilla (extensions info)
I bought and have used Appzapper when I first switched to Mac. I've used AppCleaner and AppDelete. I have Hazel which monitors the trash for deleted apps and warns of associated files to delete with the app and sometimes they find everything but neither of them find everything everytime. I found an app called Easyfind that I prefer to use to find apps and their associated files/folder to remove. Even better than Leopards spotlight search window. I really miss Tigers spotlight search window, it even searched library and system folders automatically.