Software that adds functionality to the Firefox Web browser. There are hundreds of Firefox extensions available, including toolbars, tab treatments, bookmark organizers and dictionaries.
Extensions, Themes and Plug-Ins
All falling into the "add-on" category, "extensions" add or modify the behavior of Firefox, whereas "themes" change its overall appearance and "plug-ins" add self-contained software systems such as Java, Adobe Reader and media players such as Flash and QuickTime. See browser plug-in