Perhaps it is a couple of settings in T-Bird and maybe Firefox? The 2nd post in this forums spells it out:
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.