What is the difference between a Firefox extension and a Firefox add-on?
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.
Reading Add-on (Mozilla) may help
Here is a quote from Firefox add-ons FAQ:
What are the different types of add-ons?
There are several kinds of add-ons that customize Firefox in different ways:
- Extensions add new features to Firefox or modify existing functionality. There are extensions that allow you to block advertisements, download videos from websites, integrate more closely with social websites, and add features you see in other applications.
- Complete Themes change the entire appearance of Firefox, usually including icons, colors, dialogs, and other visual styles.
- Themes are lightweight themes that use background images to customize your Firefox toolbars.
- Search Providers add additional choices to the search box dropdown. These providers allow you to quickly search any website.
- Dictionaries & Language Packs add support for additional languages to Firefox.
- Plugins help Firefox display or understand different types of media, such as Adobe Flash or Apple Quicktime.
In this context, extensions are a subset of addons.
So addons are an umbrella term for extensions, themes, dictionaries etc.