This one here: https://addons.mozilla.org/thunderbird/562/
It's kinda useless insofar as SM can already display the MUA in the
header... but some people (like me) do like spiffy pictures. :)
I'd need to get permission from the original Thunderbird extensions
author for using the icons, but the plugin per se is completely trivial
to write (at least now that I've got some idea how it all fits together).
Lemme know (on or off-list) and I'll see what the original author
says... in the worst case the icons would have to be remade from scratch.