Rated 4 out of 5 stars

I would bet than all who encounter slow start in FF 44 has also Classic Theme Restorer installed. According to my testing
Custom Buttons starts fast in FF 44 when I disable Classic Theme Restorer and vice-versa.

UPDATE: this fix slow starting of Custom Buttons:
http://custombuttons.sourceforge.net/forum/viewtopic.php?f=5&t=3656#p11038
It is not signed so possible problem in future but THANK YOU VERY MUCH ODYSEUS

This review is for a previous version of the add-on (0.0.5.8.1-signed.1-let-fixed).