Add-ons are installable enhancements to the Mozilla Foundation's projects (and compatible variants such as Portable Firefox). Add-ons allow the user to add or augment application features, use themes to his or her liking, and handle new types of content. Extensions can be used to modify the behavior of existing features to the application or add entirely new features. Extensions are especially popular with Firefox, because Mozilla developers intend for the browser to be a fairly minimalistic application in order to reduce software bloat and bugs, while retaining a high degree of extensibility, so that individual users can add the features that they prefer. (Source: Wikipedia)
From here.