Mozilla add-ons are installable enhancements to the Mozilla Foundation's projects, including Firefox, Thunderbird, SeaMonkey, and Sunbird. Add-ons allow the user to add or augment application features, use themes to their liking, and handle new types of content.
There are several types of add-ons:
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Screenshot of Mozilla Add-ons website
Type of site
|Extensions and themes hosting|
|Created by||Andy McKay, Chris Howse, Gregory Koberger, Jeff Balogh, Jorge Villalobos (jorgev), Justin Scott (fligtar), Kumar McMillan, Matt Claypotch, Stephen Donner, Wil Clouser, et al.|
|Registration||Free; only needed for developers or for special features|
The eponymous Mozilla Add-ons website is the official Mozilla Foundation repository for add-ons of Mozilla software, including Mozilla Firefox, Mozilla Thunderbird, SeaMonkey, and Mozilla Sunbird. These add-ons include Mozilla extensions and personas, but not Mozilla plug-ins.
In contrast to mozdev.org which provides free hosting for Mozilla-related projects, Mozilla Add-ons is targeted to end-users, not just software developers. Many Mozilla applications contain direct links to the website, and contain settings to poll for updates to the extensions and the application itself. Firefox 3 includes features for retrieving and displaying content from the website in the add-ons manager.
On January 30, 2008, it was announced that over 600 million add-ons had been downloaded from the site and that over 100 million add-ons automatically check the site for updates every day.
On July 26, 2012, Mozilla announced that 3 billion add-ons were downloaded from the site.
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.
Extensions can be developed using the following technologies:
Unlike Google Chrome, the Mozilla platform has no mechanism to restrict the privileges of extensions. Extension code is fully trusted by Mozilla applications. There are no restrictions on intercommunication between extensions as well as the operating system. This means that one extension can read or modify the data used by another extension or any file accessible to the user running Mozilla applications.
Mozilla extensions are per default installed into the applications user profile where their code can be overwritten by the user or any program run by the user. Since the Mozilla platform does not check the integrity of installed extensions they can trivially be (ab)used for arbitrary code execution.
From Firefox 40.0, Mozilla began to roll out a requirement for extension signing in the Release and Beta channels to improve end-user security. From 40.0, the browser warns the end user an extension is unsigned, from 43.0, unsigned extensions could only be installed if a special option in the about:config page was enabled. In Firefox for Desktop 48.0, unsigned extensions are blocked in Release and Beta versions without override, but Firefox for Android still allows signature enforcement to be disabled. The Developer Edition and Nightly versions of Firefox will have a setting to disable signature enforcement. An unbranded version of Firefox Release and Beta is planned to allow developers to work on extensions without the requirement. The first Firefox ESR release to enforce signed extension is 52.0.
Extensions contain XML files bearing metadata utilized by the mechanism which controls add-on installation. Among other things, this file identifies maximum and minimum versions of a Mozilla project application on which the add-on may be used. If an attempt is made to install the add-on on a version outside of this range, it will install but will be disabled. The success of a formal compatibility check is no guarantee the add-on will work, however. It is even possible to override the compatibility check using various extensions.
The add-ons manager periodically checks for updates for extensions installed from the Mozilla Add-ons website, although checks for updates can be manually initiated by the user. If the developer includes provisions to check elsewhere, the add-ons manager will do so.
Themes, originally called Personas, allow users to quickly change the look of their applications. Although support for Personas was initially available as an extension, the feature was built into Firefox 3.6, and later other applications. These background themes differ from traditional themes available before, which were implemented as Mozilla extensions. Mozilla themes are more basic, easier to create, and easier to install, but are limited to changing the background image, background color and text color of toolbars, menu bars and status bars. They cannot alter the shape or appearance of toolbar items or tabs in the way that extensions can. In regards to installation, they can be installed and activated with exactly one click.
Originally hosted on GetPersonas.com, they were moved to the Mozilla Add-ons website in 2013. In addition, because to the similarity of the name with an authentication scheme called "Mozilla Persona", Mozilla stopped using this term in favor the more generic word "theme", even though there is a distinction between theme implemented as an extension and Personas, in terms of installation and handling by Add-ons Manager.
Although the feature was met with general favor, TechCrunch has criticized Mozilla for focusing on developing eye candy for the browser rather than placing more attention on improving the speed and usability of Firefox.
Mozilla plugins are browser extensions used to render web contents that the application itself cannot render. They are created using NPAPI framework. Firefox 41.0 comes with two plug-ins pre-installed: "Open H.264 Video Codec" by Cisco and "Primetime Content Decryption Module" by Adobe Systems. Common third-party Mozilla plugins include Adobe Flash Player, Acrobat Reader, Adobe Shockwave Player, Java SE, QuickTime and RealPlayer. Unlike other types of add-ons, Mozilla does not distribute plug-ins, and the add-ons manager cannot update them.
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