Firefox 2 and Gecko 1.8 End of Life
vm writes "According to Mozilla and other sources, Firefox 2 and Gecko 1.8 will soon be left behind some time in mid-December. The end result: no future security or stability updates. This will affect Thunderbird 2, SeaMonkey 1.1, Camino 1.5, and any other projects based on Gecko 1.8. So, if you haven't already upgraded, there's no time like the present."
See the mozilla.dev.planning thread in which the Firefox 2 EOL was first discussed. Yes, just dropping support altogether would cause problems for products like SeaMonkey, Thunderbird e.a. (which haven't yet shipped a version based on 1.9), and that's why that won't happen. Firefox 2 might be unsupported, but necessary security fixes will continue to be identified and backported to the Mozilla 1.8.1 branch, so that those products can continue to release security updates for a while until after their next versions have shipped (hopefully by the end of Q1 2009).
Thunderbird 3 is under development. An alpha version is available.