Hi kylehuff,
I join my welcome message do djohnston's.
I have pointed to the present thread at kylehuff's code google issues section when I posted there, this is how he has known about our forum. And I started being interested about WebPG after Goodbox, who just provided me a webspace at the new crunchbang-fr.org web server, told me he didn't use a mail client, and he needed to send me codes and passwords in a safe way to make me access to the webspace. Finally, he sent me an encrypted text file as an attachment, however in the meanwhile I decided to put interest in kyle's program and offered to continue testing it and reporting about my findings, because it is the only plugin for webmail I know of which can allow an easier use of gpg for people who don't use the command line too much. (I found another one but according to the comments at the mozilla plugins place it is crummy).
About Thunderbird : there is already Enigmail which has been allowing to use easy GPG encryption. I have started with it long ago. I hope it still works well. When it comes to webmail it is different because there is no other good tool for this purpose.
Right now I am using inline WebPG toolbar in the present post. It forces me to use secure editor, which does not provide any of the usual buttons from the editor, and forces me to sign the message (or select any other action among several... even though I can remove the signature), before clicking "Insert".
It would be nice to have an additional option allowing to deactivate this feature in forums and any place which is not a webmail. But how does this feature recognize which type of edition window is used? Maybe asking the tool to be active only on some domains would be a better idea?