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.

Firefox also supports a variety of themes/skins for changing its appearance. Themes are simply packages of CSS and image files. Many themes can be downloaded from the Mozilla Update web site.

The change of default theme from Qute to Winstripe in Firefox 0.9 was subject to vocal debate. The Winstripe theme was created by heavily modifying Pinstripe, a theme designed with Mac OS X in mind. Prior to that, Firefox and its predecessors had used the Qute theme, designed by Arvid Axelsson. Due to licensing issues, the theme was prevented from being released under the Mozilla Public License. Axelsson was upset about being notified about the theme change only a few days before it took place, and posted the transcript of his dialogue with Ben Goodger, who had informed him of the change, on the MozillaZine forums, breaking the news before it was formally announced. Although many people criticized the new theme when it was rolled out, eventually the tension subsided. Axelsson continues to produce Qute privately. Axelsson still makes Mozilla Thunderbird’s default theme.

View Mozilla themes

Dictionaries

Another type of XPI add‐ons for Firefox — dictionaries for spell checking of input fields.

Plugins increase your browser functionality like viewing special graphic formats or playing multimedia files. Plugins are slightly different from extensions, which modify or add to existing functionality.

Firefox supports plugins based on Netscape Plugin Application Program Interface (NPAPI), i.e. Netscape-style plugins. As a side note, Opera and Internet Explorer 3.0 to 5.0 also support NPAPI.

On June 30, 2004, the Mozilla Foundation, in partnership with Adobe, Apple, Macromedia, Opera, and Sun Microsystems, announced a series of changes to web browser plugins. The new API will allow web developers to offer richer web browsing experiences, helping to maintain innovation and standards. The new plugin technologies are expected to be implemented in the future versions of the Mozilla applications.

Mozilla Firefox 1.5 and later versions include the Java Embedding plugin,[5] which allow Mac OS X users to run Java applets with the latest 1.4 and 5.0 versions of Java (the default Java software shipped by Apple is not compatible with any browser, except its own Safari).

View Firefox Plugins