Rated 5 out of 5 stars

This is a neat little tool :-) I've already found a use for it twice.

However I'm very new to this kind of thing and could really do with a manual or at least a few more details on usage, e.g. how to choose a different icon, how to use the fields in the New Button dialog, etc etc.

Very nice anyway, five stars from me.

This review is for a previous version of the add-on (0.9.6.1-signed). 

Some more info...

button id is just a unique string...
menu/context (fairly tricky) see notes...
icon - any link to resource will do (http/chrome) but if the resource is unavailable (because the add-on is disabled for ex..) you'll get blank button. so the best way is to convert a 16x16 icon to data:image/png;base64...
you can install killspinners add-on and check the samples as an example of for menu, icon usage