How to shoot yourself in the foot...

How to shoot yourself in the foot in a browser extension environment (let's say... Firefox)?
You first device a niiiice gun overlay, in XUL, styled with CSS, with a trigger interface in like 346524563425 languages using the wonderful firefox i18n mechanism, you connect your gun to some serious killing machine (preferably written in Java) and then, when you're finally done with all these and you shoot your gun:

Security check failed: permission denied to access 'body.lowerbody.foot&side=left' from content at "chrome://gun/content/bigshotgun.xul".