Note that InstandBird last had a release in December 2013 and their blog was abandoned in July 2014, so their future is a bit up in the air even though commits still happen on the Mercurial repository at Mozilla. Pidgin has released multiple security updates since the December 2013 release of InstantBird 1.5. I'm unsure if the Tor folks have incorporated these changes offhand. They use the clunky gitweb client for web browsing of their repository and I don't have time to clone a git repo at the moment.
It's also worth noting that Pidgin's libpurple uses XMPP for Facebook and Google chats, neither of which will work well any longer due to both of them deprecating XMPP.
If you'd like to use secure chat and are unconcerned with the meta-data, there are quite a few better options at present. If you'd like to use secure chat over Tor, you can use any of those options over the Tor network manually. This does have a lot of promise for making it easier for a regular end user, though, so testing and feedback are encouraged.
*Update - According to a comment reference below, they now build without libpurple using their own custom Javascript libraries.