Rated 5 out of 5 stars

This is so good I'm retiring my "Flip or Rotate Image" extension (and pointing people that find it to here), which used the CSS transform method you cleverly avoided, and doesn't have the brilliant EXIF detection of yours.
If you added a transition/animation of the rotation and made the menu show up in Private Browsing windows I'd be completely done :)

This review is for a previous version of the add-on (1.0.1). 

Thanks for the kind words :) I'll file both of the improvements you mentioned as Issues on github. The private browsing one is probably trivial, and the transition would be a neat twist I'd be happy to add.