Topic: I've found why the Gmail's buttons don't appear sometimes.
It is because the last version of FireGPG does not work with the newer Gmail user interface!
If I change back to the older version of the user interface and compose a message, the buttons appear!
The option to change the user interface is located at the upper right corner of the Gmail menu.
Guys, please try and see if this solves your problem.
For some reason, FireGPG does not work with the new Gmail's user interface .=D
Regards,
Leo.