See also: Firefox Extension system and List of Firefox extensions
Firefox users can add features and change functionality in Firefox by installing extensions. Extension functionality is varied; such as those enabling mouse gestures, those that block advertisements, and those that enhance tabbed browsing.

Features that the Firefox developers believed would be used by a small number of its users have not been included in Firefox and left to be implemented as extensions. Many Mozilla Suite features, such as IRC chat (ChatZilla) and calendar have been recreated as Firefox extensions. Extensions are also often a testing ground for features that are eventually returned to the main codebase.[citation needed] For example, MultiZilla was an extension that provided tabbed browsing when Mozilla lacked that feature.

While extensions provide a high level of customizability, PC World notes the difficulty a casual user would have in finding and installing extensions as compared to their features being available by default.

Most extensions are not created or supported by Mozilla. As extensions have the same rights to the user’s system as Firefox itself, it’s possible to create malicious extensions. Mozilla provides a repository of extensions that have been reviewed by volunteers to not contain malware. Since extensions are mostly created by third parties, they do not necessarily go through the same level of testing as official Mozilla products, and they may have bugs or vulnerabilities.