Talvez seja um par de configurações no T-Bird e talvez no Firefox? O segundo post nos fóruns explica:
In Firefox, go to View menu, select Page Style, and make sure "Basic Page Style" is selected, NOT "No Style".
In Thunderbird, you'd go to View menu, select Message Body As and make sure "Original HTML" is selected.
These are changes Enigmail ask you if you want it to make during the Setup Wizard process.