Topic: Don't reinvent the wheel: use Enigmail
This is more of a curiosity for why you have chosen to do things this way, as opposed to a request to change them.
I was wondering why you don't expand on the Enigmail Thunderbird extension to work with Firefox rather than starting from scratch? Enigmail already has all the routines for connecting securely to the gpg program, all it needs is the interface to work with firefox. Instead of redoing all this, why not make use of the mature and secure Enigmail routines, and just focus all your attention on the Firefox/Gmail user interface? I think the end result would be a better extension (though I'm sure you have your reasons for starting from scratch).
There's even a 3rd firefox extension with similar functionality: Enigform. I even found a post on the Enigform authors site suggesting that 'Enigmail, FireGPG and Enigform's (myself) authors should "sit down" and discuss [...] a standard key-management interface (Enigmail's, for example)'. Have you heard from him about this? Here's the post: http://foros.buanzo.com.ar/viewtopic.php?f=37&t=231