Inform Torbutton users about TBB
Once TBB has a way of telling users to upgrade properly (#2285 (moved) and friends), we should be more aggressive about telling regular Firefox users that they are out of date. The easiest way to do this is to pop up a XUL dialog that tells Torbutton users where to find TBB. Ticket #3811 (closed) created the string for this dialog window.