FullerScreen 2.0 was incompatible with Gmail Manager. It's partly our fault, partly Gmail Manager's. Let me explain... We have some utility extensions to Array that we coded extending Array.prototype because we found them useful to anyone. But Gmail Manager does a for (var email in this._accounts) where this._accounts is an array... So it saw our extensions to the prototype ! Let's call that bad practice on both sides.

Anyway, it's fixed now, and we have a 2.01 available from our site and already submitted to addons.mozilla.org.