Firefox fails to start if Torbutton is enabled and the network is inaccessible.
If Torbutton is enabled when Firefox boots, it makes two requests to check.torproject.org through Tor. If this request fails, Torbutton will simply try again, and again, and again, and again… Until a successful request is made, Firefox will not start.
This causes two major problems. The more common and less severe one is that it dramatically increases Firefox's startup time. The second is that if the user is connected to a network that requires them to perform some action through a web interface (e.g. logging in or accepting a ToS agreement), they'll find themselves in a perverse, Freenode-esque catch-22 where they can't connect to the network unless they disable Torbutton, and can't disable Torbutton until they connect to the network.