Both FireGPG and PGP Desktop/Mail.app do not see my signed test messages as valid. FireGPG in Gmail produces the error "The first signature of this mail is not valid!", PGP Desktop/Mail.app just reports "* PGP Bad Signature, Signed: 04/13/07 at 08:33:39". Encryption seems to work fine, producing messages both can open.
FireGPG 0.3.3
GnuPG 1.4.6
Firefox 2.0.0.3
PGP Desktop 9.5.3
Mail 2.1.1
I don't see a howto or other instructions on the site, so basically I've been testing by selecting the entire contents of the message window and using the Sign/Send button. If I put the same text into an ascii file and do a gpg --clearsign and gpg --verify on the output it shows a good signature.