Mozilla releases a Thunderbird Stable and ESR version every six weeks just like it does for the Firefox web browser. While that makes sense for actively developed products that get new feature additions regularly, it does not really make a difference in regards to the Thunderbird email client anymore as it does not really receive new features regularly anymore.
The difference between stable and ESR releases is that Extended Support Releases are more restricted in regards to updates they receive. While they do get all the security fixes and such, major changes to the program's core are not added directly to them.
These changes are only rolled out every eight release cycles equaling every 54 weeks. The idea behind this is to give Enterprise and business users enought time to adapt to the changes before they are rolled out.
Thunderbird development on the other hand has been put on hold more or less, which means that the stable and ESR versions have been identical in the recent past.
That's the core reason why Mozilla made the decision to merge the two release strains. You are probably wondering how this looks like:
- Mozilla will release Thunderbird 24 to Mainstream users (stable channel users) and Thunderbird 17.0.9 ESR to Enterprise users on September 17th.
- On October 29th, Mozilla will release Thunderbird 24.0.1 to mainstream users and Thunderbird 17.0.10 ESR to Enterprise users.
- On December 10th, Thunderbird 24.0.2 will be released. At this day, Enterprise users will receive an update route from 17.0.10 to 24.0.2 effectively merging both channels so that they run in sync from that time on forward.
You have probably noticed the use of three digit version numbers in the announcement. Mozilla announced plans back in May to change the version numbering of ESR releases. The basic idea here was to use the second digit instead of the third for the releases, and the third for smaller updates and security fixes.
It is therefore very likely that Thunderbird 24.0.1 and 24.0.2 will become 24.1 and 24.2. That has not been confirmed though yet by Mozilla representatives. (via Sören)