Mozilla has NOT dropped support for Thunderbird, they simply won't be adding any new features to it except security updates

Mozilla has only said that their developers will not be working on Thunderbird further, i.e. adding new features, etc after 20 Nov 2012. They will continue making security updates available for Thunderbird until 30 December 2013.

At the same time, outside developers ("the community") can continue working on a parallel version of Thunderbird (still released by Mozilla) which will include new features, etc.

Considering that Mozilla.org involvement in developing Thunderbird will stop only after Quantal is released, it's a little early to begin looking about for alternatives. Even if the community doesn't pick up the slack and Thunderbird begins to go obsolete, I'm sure that Canonical will have an alternative in place by the R-series (April 2013).