It's well designed and easy to use which is all I could ask for for an app like this. Thanks for sharing!
Two things I've found that are both minor so I'm not "docking points" ;)
>Click the Hibou highlight button. It will be saved here until you make a highlight.
I think you mean "Highlights you make will be saved here."?
And if I activate Hibou on a page, I can't seem to disable the highlighter without refreshing the page. Pressing Alt+H doesn't get rid of it and neither does clicking the button to deactivate the highlighting cursor. This is an annoyance, but it's unlikely that I'll ever press Alt+H without the intent of highlighting something - so isn't an issue I'd run into day to day.
Clicking onto the page to close the app dialogue and then re-clicking the icon to bring open the options shows that the highlighter reactivates itself. Upon reactivation or clicking the bubble I expect it to remain disabled until I activate it again.
http://i.imgur.com/SQGXKbI.png
Lastly - it should be clarified what benefits "Logging in" gives. How does it make Hibou "even better"?