the_glu wrote: I don't understand, FireGPG shouldn't insert anything anywhere...
It shouldn't, but it apparently does.
I'm going to do more experimenting, but there is definitely some sort of corruption visible on decrypt that's causing errors with the receiving script. With other encryption shells, this has typically been some sort of leading linefeed or something that isn't obvious when just looking at the decrypted plaintext. It is conceivably something in the webmail site's processing of the outgoing message, but whatever it is, it seems to be limited to FireGPG messages.
I don't encounter this when using GPG outside FireGPG to prepare the messages. Even when they are prepared in the FireGPG text editor and cut & pasted in the outgoing message they fail for some reason.