The shift to a new release process (the 'Rapid Release Process') has been difficult for organizations that deploy Thunderbird and Firefox to their users in a managed environment. These groups — which include small & medium business, enterprise, academic, and government — want to continue to offer Mozilla products to their users, but they need a version of Firefox and Thunderbird that gives them a longer support tail than what we currently offer. Mozilla is therefore proposing an Extended Support Release (ESR) scheme for Firefox as well as for Thunderbird that will help meet those needs.
Since Thunderbird is a significant downstream consumer of the Gecko platform, which is itself influenced by Firefox's plans with respect to security & maintenance policies in particular, a large portion of the Thunderbird's own ESR proposal will be derived from Firefox ESR proposal. However, while sharing technology, Thunderbird is a distinct product which is exposed to different distinct security and market environments, and we don't want to assume that the discussions which have focused on Firefox necessarily apply as-is to Thunderbird.
We intend to conclude on this subject within two weeks in order to be able to offer Thunderbird ESR 1.0 at the same time that Thunderbird 8 or 9 is released. Thunderbird 3.1 will be declared end-of-life 12 weeks after the release of Thunderbird ESR 1.0.
We would like to encourage Thunderbird users to read the Firefox proposal, discuss its relevance for Thunderbird and express the differences that may apply.
The Firefox' proposal is documented here. The discussion on Thunderbird ESR proposal should happen in the Thunderbird Enterprise mailing list.