Hello,
I'm not sure it is done in purpose, but firegpg decrypt or confirm signature of emails (using gmail) only it the email is set as "unread" before opening it.
So the first time it works fine (firegpg decrypts the gmail email directly) but if I go back to this email later, I see it directly undecrypted (I see it as if there is not firegpg plugin, (I see BEGIN PGP MESSAGE and so on).
I order to decrypt message in easy way, I found that I can put message "as unread" (still with gmail), and then re-open it. In that case, firegpg decrypt it as it should.
Using Ubuntu9.04 ( gpg already installed : (GnuPG) 1.4.9) and Firefox 3.0.11
Best regards
LouiZo
Update:
If I have a message that is not decrypted (because I already read it before, so it is not marked as "unread") , and do a "reply" on it, then it is decrypted by firegpg (I can see decrypted message above, but I still have the crypted message BEGIN PGP MESSSAGE on my reply text...)