Is there a way to use vim/gvim as an editor for thunderbird? There was an add-on for it but it is now very out of date.
You could look into the Teledactyl add-on for Thunderbird from 5digits.org. They produce a Pentadactyl add-on for firefox which works nicely for controls, although text boxes are admittedly un-vim-ish.
Feature-list says it supports external editors, so gvim could be in your future.