I have developed a couple of extensions for Firefox, and am annoyed that it is so hard to get the extension signed. When an extension isn\'t signed, it says \"Author not ve
Tucows sells Comodo code signing certificates for $75 per year, that's as cheap as it goes from what I can tell (https://author.tucows.com/, "Code Signing Certificates" section). That's still too much money for me to spend so I didn't try how it works. Not that I can try, from what I can tell you need to be a registered organization to buy a Comodo certificate.
As to Ascertia, getting a certificate is easy enough (http://www.ascertia.com/onlineCA/Issuer/CerIssue.aspx) - but such a certificate is as worthless as a self-issued certificate because you would need to import their root certificate before seeing an effect.