Usability Rated 4 out of 5 stars
This is a good idea, and makes fullscreen browsing easier. By good I mean that one of the flaws with the Bookmarks Toolbar has been lack of keyboard accelerations, you can't even TAB to it from the address bar...
Two options that I think would make your addon GREAT would be:
1. An ability to change the root (my bookmark ontology is kept in Bookmarks Toolbar, except the Unsorted).
2. An option to open bookmark in New Tab
#2 being pretty essential in my opinion, as the user must CTRL-T/N before opening the bookmark. I notice pressing CTRL-ENTER on an item will not perform similarly in the context menu (or toolbar) as the Bookmarks Sidebar, making this an unexpected point of frustration.
Nice work, I hope you consider my niche ideas!