If you’ve had issues trying to get Thunderbird to send your PGP signed e-mail using anything other than SHA-1 there is a quick and easy fix that will let you pick whichever hash you prefer.
Open up Thunderbird’s preferences
On the Advanced Tab, under General click Config Editor…
In the about:config window search for “extensions.enigmail.mimeHashAlgorithm” without quotes. Double click on this and enter a value. The value will determine which hash algorithm is used for signing.
The values are as follows:
0: Automatic selection, let GnuPG choose (note that while this may be the default it may also be the one that doesn’t work depending on your configuration).