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  There was an add-on for it but it is now very out of date. 
Using vim/gvim as editor for Thunderbird
vim;gvim;thunderbird
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.