There is currently a problem that Enigmail, the OpenPGP extension for Thunderbird doesn’t work with Gpg4Win. The latter is the GUI version of GnuPG for Windows. The versions tested were Enigmail 0.95.1 and Gpg4Win 1.1.0 but I understand other versions are affected.
There seems to be some dispute as to which program is at fault and the most seen recommendation is to un-install Gpg4Win and install the plain command line version of GnuPG. Although there is some overlap, both provide a key management GUI for instance, this would lose some of the useful disk management functions of Gpg4Win.
I have discovered that there is a much easier fix. In Thunderbird, go to the OpenPGP ==> Preferences menu item and in the “Files and Directories” window, tick Override and enter C:\Program Files\GNU\GnuPG\gpg.exe
. Now stop and restart Thunderbird and every thing works just fine.