I will be honest, I run a LOT of addons, that is what is great about Firefox.
I have used FireGPG for a while, and it has never integrated with Gmail (I've never seen the buttons, etc). I always assumed it was an addon conflict, but never bothered to troubleshoot it.
I finally took the time, and found out that the conflict is with Header Monitor:
https://addons.mozilla.org/en-US/firefox/addon/575
I have no idea WHY this would cause a conflict (it was actually one of the last ones that I would have guessed, especially since the parent plugin LiveHttpHeaders doesn't cause any problems) but for some reason, if I enable that, I no longer get the encrypt buttons in FireGPG
Anyway, I don't know whose "fault" it is, but if FireGPG developers can work something out, it would be great.
Header Monitor can be quite handy.