The word Tinderbox when referring to the application is, by common usage, often shortened to 'TB' in discussion in places like the user-to-user forum. This use of 'TB' is not an acronym. Rather, it is just a simple shorthand for those who find they need to type the word Tinderbox a lot!
It is also worth noting that the correct spelling of the application name is 'Tinderbox' and not 'TinderBox'. The latter is sometimes assumed from seeing the TB 'short' name.
'TBX', the file extension used by Tinderbox file, is also used as a shorthand for a Tinderbox document(s).