Rated 5 out of 5 stars

I like it! One minor thing. The Firefox button still is capable being slightly smaller in size by reducing the size of extra space in height and width imo.

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

The appearance of Firefox Button depends on the theme you are using. You may use userChrome.css to embellish it with ideas of your own.