As mentioned today on our Firefox Blog, support for Firefox 3.6 will be likely ending in late April 2012. However, the Enterprise/Thunderbird/ExtendedSupport has not been refined yet. It still states,

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.

Given that Firefox (though not yet official) has decided to make Firefox 10 the first ESR, it would seem logical that Thunderbird would follow suit with Thunderbird 10 (release date January 31, 2012). Allow for the 12-week ‘grace period’ and you end up with April 24th, 2012. However, Thunderbird lagged behind in regards to the Rapid Release Process. The first release to come out of the Rapid Release Process was Thunderbird 6.0 back in August 2011. Also keep in mind, the next major version after Thunderbird 3.1 was Thunderbird 5.0 released on June 28, 2011. If Mozilla had followed the original six-months from release of next major version, Thunderbird 3.1 EOL would have been in December 2011.