Hello,
I just tested out the "Decrypt content when reply" feature - it looks like the quoted decrypted text is put outside of the scope of subsequent encryption. So, essentially, when I receive an encrypted email, I "reply with decrypted" and firegpg encrypts only my reply, but not the quoted text. The quoted block is appended to the email in cleartext.
When I made my initial feature request this is I had in mind: I receive an encrypted email, I hit reply, firegpg quotes the content of this email in cleartext, I type in my response and sign/encrypt/send it. The entire email, including the quoted bit should now be encrypted.
Please let me know if this makes any sense to you at all 
Do you guys have an IRC channel, by any chance?