Thanks... there's been some progress 
The command:
--comment \"Public key available at: http://www.syntilect.com/cgf/publickey\"
produces:
Comment: \\Public key available at: http://www.syntilect.com/cgf/publickey\
as output.
So I guess that something funny is going on with how the strings are escaped and unescaped. I've tried dotting extra backslashes around, but no luck so far. Similarly, the command:
--comment \"// \t \t Public key available at: http://www.syntilect.com/cgf/publickey \t \t //\\\"
produces:
Comment: \\// \\t \\t Public key available at: http://www.syntilect.com/cgf/publickey \\t \\t //\\
as output.
Finally, trying something like --comment \"..." was a bit of an exercise.
This produced a crash that requires a complete restart of FireFox. Once you've done this, Firefox is fine, but you can't modify any of the options for FireGPG, and so I couldn't remove the offending extra command line option... Restarting FireFox again doesn't resolve the issue, neither does uninstalling and then reinstalling FireGPG. The eventual solution was to manually edit things in about:config.
So I'll file this as a bug, although it might be a couple - the escaping/unescaping probably isn't major, the other one who knows
Also, I've noticed that whenever you put in an extra command line argument that makes FireGPG barf then FireGPG loses track of the preferred private key for signing, and you can't choose it in the options window because it (they all, I only have one private key on my computer) disappear. Restarting your browser puts it back in the list though so it's not a problem really
Christo