Thanks all for your replies and suggestions. I have already implemented a few of them and v1.0 should be even cooler than it already is. Two of the things I implemented :

  1. a pref allowing to show the statusbar when the pointer is hovering over an hyperlink
  2. a pref controlling an "even fuller mode" hiding the vertical scrollbar on the document ; we get closer to a real slideshow mode

Stay tuned, and thanks again.

On a side note, I uploaded Fuller Screen on addons.mozilla.org and nominated it for public view. Please consider helping it reach public view if you like this extension, thanks.