I'm experiencing the exact problem that Arizona originally reported. I have 2 gmail accounts primary and secondary. Secondary forwards to Primary, so I only check primary. I have FireGPG setup to send e-mails for me as secondary. I'm not sure if any of that matters.
I sent an e-mail signed (then tried again encrypted) to my primary account. Since FireGPG sends using my secondary account, it arrives addressed to primary, from secondary, with signature.asc (or encrypted content). In both cases FireGPG reports "no signed or encrypted content found". :-/ I'm in the process of having someone else send me signed or encrypted content.