Mac OSX mistakes Tor as Firefox default browser
If you use Mac OS X and have Firefox set as your default browser the OS will mistake Tor as Firefox.
Why this is a problem should probably be obvious but here is an example: Let's say you have Tor open but not Firefox. If you click a link in any program (e-mail, Word Doc, PDF, etc.) outside of the Tor browser your link will open in Tor Browser instead of Firefox.
If you were to click a link in an email from Facebook while you are using Tor you will have just sent a request to Facebook with identifying info about your real identity.