The Tor Browser Bundle (TBB) uses Mozilla Firefox ESR.
Mozilla will offer an Extended Support Release (ESR) based on an official release of Firefox for desktop for use by organizations including schools, universities, businesses and others who need extended support for mass deployments.
Since TBB has the goal to be secure and stable, it uses the ESR version, not the latest and greatest Firefox. The TBB is regularly updated with the latest version of Firefox ESR.
About the difference to the "normal" Firefox:
Users who want to get the latest features, performance enhancements and technologies in their browsing experience should download Firefox for personal use, as these improvements will only be available to ESR users several development cycles after being made available in Firefox for desktop.
On August, 12th 2014 Tor Browser 4.0-alpha-1 was released. Part of the release notes for this version is:
- Update Firefox to 24.7.0esr
This is an example of how the Tor project keeps track of the latest versions of Mozilla Firefox ESR.
Mozilla Firefox ESR 31.0 is too young to be included in the TBB. It is not out of the "qualify" phase for a new ESR. See the life cycle chart at Mozilla.