Rated 1 out of 5 stars

This can be done by default in firefox.
Edit,Prefrences,Content, Fonts and colors, Advanced, Minimum font size.

This review is for a previous version of the add-on (2.1). 

Sure, this is a simple add-on, but it provides the convenience that is lacking from the default interface, especially by providing a switch function. It is often the simple add-ons that do best :)