Almost perfect for me. No matter license, Icon&UI, features and customizability.
The only thing I mind is that it requires so many blanket permissions by default, which seems not be good practice. It's better to request permission separately when user does want to enable some feature that needs corresponded one (see the extension 'SiteDelta Highlight' s settings page for example).
(That popup little button is mooooooooeeee!!!
Maybe it's better if could be set to click to show up panel? As now it's hover-by showed up, but seems no option to change.
On-demand permission is surely better. It's just too much trouble for me to implement it. Not worth the time. Anyway, it's open sourced and there are reviewers in Firefox team who would actually build the source code and verify the product.