Topic: OpenPGP embedding
I think it would be more comfortable to have OpenPGP directly implemented into the extension.
For example by adding the necessary files from the GnuPG project (gpg.exe, etc.) directly to the extension, if you would change the license of FireGPG to GPL. So that it is not necessary anymore to download and install any additional programs like GnuPG (especially useful in environments, where it isn't possible to install additional software).
Maybe you could offer a lite version (not included) and a full version (included) of FireGPG then.