Since you appear to want to learn, I won't feel bad (or pedantic) in explaining that he installed an extension, which is a browser add-on. A plug-in is also a browser add-on, but is used to describe QuickTime, Flash, Java, etc.

After I wrote the above, I double checked myself and, apparently, I am using Mozilla's definition. However, it seems Apple also differentiates among the terms add-on (the super-set), and extensions/ plug-ins (subsets).