Rated 4 out of 5 stars
This was one of the main features keeping me on Chrome, so I'm *thrilled* to see it added to FF. It works well, though it took me awhile to realize that it had worked- there was no indication that anything had happened when I clicked the icon, but it did add a desktop file to ~/.local/share/applications (on Linux).
I would love to see the ability to use stylesheets. I use a lot of userstyles for apps like Twitter to make it more conducive to using as a desktop app. Looking forward to further development.
Edit: I'd also like to see the menubar done away with, since it doesn't really seem to serve any purpose other than a "quit" and a "select all" option.