Thunderbird supports extensions
I’ve been pretty unexcited by Thunderbird so far: I’m pretty picky about my mail clients (so far Evolution is the only client I’ve really liked) and Mozilla’s never really lived up to my expectiations. I’m preparing to change my mind now that Thunderbird supports extensions. Firebird’s extensive collection of extensions is one of its strongest advantages, and the idea of a mail client that can be customised in a whole bunch of different ways by adding new extensions is pretty inspiring. I might even have to try and learn to write some myself.