From what I read in the internets, there was a time, when gnome-extra/nautilus-sendto had a USE-Flag thunderbird to enable sending mails via thunderbird in the nautilus context menu. This USE-Flag is no longer, but I would like to hereby request it! Reproducible: Always
It looks like the feature for mail clients is actually all in the plugin named evolution..., try building with USE="evolution", it should unlock the thunderbird sendto. However, don't start asking us to make evo support optional as the code indeed requires evo at build time at least, forward this request to upstream so they split the generic part of the code to another plugin.
confirming previous comment.
