Same here:
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3
FireGPG 0.7.10

checkDock/compose chrome://firegpg/content/Webmails/Gmail/cgmail2.js 669 children[0] is undefined

I was able to encrypt/decrypt plain messages, but encrypting and attaching a file didn't work.
Restarting Firefox resets FireGPG it seems, but of course I still cannot attach encrypted messages.

/Zane