It would be incorrect to say that Thunderbird is dead. Rather, Mozilla appears to have moved the desktop email client from the hospital to hospice, and it's now up to the community to decide just how development on the app should continue -- if it's to continue at all...

Although desktop clients used to be the sensible choice for heavy email users, robust services such as Google's have made Web solutions completely viable. Nonetheless, after relying on Gmail for about eight years, I recently switched to a local application (Postbox)...