Rated 4 out of 5 stars

Now we have an alternative to Stylish! This addon stores style as individual .css text files so you can back them up easily. Looking forward to the integration with userstyles.org web site, and an icon on addon bar. Thank you Girish Sharma, and please, please never stop developing.

This review is for a previous version of the add-on (0.1.1-signed). 

Yeah, much to anticipate in coming weeks. :)
Version 0.4 has the button which you can place anywhere you want, and also a Firefox Menu entry to quickly enable/disable or to manage your styles.