Firstly, I would just like to reiterate what other people are saying - thanks for a great utility!
I have also been experiencing this problem. When I look at a signed message in my Sent Items, it shows as a valid signature. When somebody replies to it (they don't sign their replies), it reports that the signature is invalid. I have used GMail's "Show Original" function have have noticed that there's a difference between what's in the message I sent and the message that comes back - some extra characters have been added (though they don't display in the normal view).
This is the original message I sent:
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
I've just sent this back as normal (unzipped). Somehow I managed to
completely forget about it!!
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.7 (MingW32)
iD8DBQFIHJTs3tCzz5kzEZQRAkZWAJ4omBNV3SBlRjXJ24wpYlPHNlOIAgCaA9rB
t9LT06PFTw748c02zbA2+Ik=
=opoq
-----END PGP SIGNATURE-----
This is what it looked like when it came back in a reply:
-----BEGIN PGP SIGNED MESSAGE-----=20
Hash: SHA1=20
I've just sent this back as normal (unzipped). Somehow I managed to
completely forget about it!!
-----BEGIN PGP SIGNATURE-----=20
Version: GnuPG v1.4.7 (MingW32)=20
iD8DBQFIHJTs3tCzz5kzEZQRAkZWAJ4omBNV3SBlRjXJ24wpYlPHNlOIAgCaA9rB=20
t9LT06PFTw748c02zbA2+Ik=3D=20
=3Dopoq=20
-----END PGP SIGNATURE-----=20
This second version obviously wouldn't verify. I copied it into Notepad, removed the extra characters (including the spaces at the start of the lines) and it verified successfully.
If FireGPG could identify and remove these extra characters before attempting to verify the message, I believe it would then show (correctly) as a good sign.