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Blocking By Domain

Plugins such as Flash are a performance and security problem for Firefox users. However, some legacy Flash content hasn't yet been ported to HTML and is valuable to users. Rather than block plugins entirely, Firefox is limiting the impact of plugins by blocking certain domains from using plugins. These blocks improve Firefox security and performance also make the click-to-activate feature more valuable to users by reducing unnecessary prompts.

According to the Firefox plugin roadmap, the plugin blocklist list is planned for Firefox 55.

Effects of Plugin Blocking

Once a site is included in plugin blocking, it is not possible for that site or any subframes within that site to use plugins. Any attempt to use a plugin (via the <object> or <embed> element) will behave as if the plugin was not installed, and use fallback content as specified by the HTML standard. Plugin detection mechanisms such as navigator.mimeTypes and navigator.plugins will also behave as if the plugin were not installed.

There are two versions of plugin blocking:

First-party Plugin Blocking List
Sites on this list may not use plugins.
Third-party Plugin Block List
Sites on this list may not use plugins when they are loaded into a third-party iframe in another site.


in order to improve Firefox security and performance, there are two major categories of sites Mozilla may choose to add to the plugin domain blocking:

Sites commonly embedded in a 3rd-party context
Web sites which are commonly embedded into other sites using iframes can have an outsize impact on browser security and also cause many sites to show plugin activation prompts. Sites in this category may include advertisers, social media sites, and other utilities.
Top sites
Many popular sites work properly without plugins, but still prompt users for activation. In order to improve site performance and reduce the number of plugin prompts that users see, popular sites that show unnecessary plugin prompts or slow the browser down with unnecessary plugins will be added to the plugin block list.

List Contents and Updates

The plugin blocking lists are freely-licensed lists maintained by Mozilla in github. The initial contents of the third-party plugin blocking list are based on based on Disconnect's open source blocklist. Subsequent updates to the list are being made using telemetry from Firefox experiments. In general, sites will not be removed from the list unless there is an error that causes user-visible and serious site brokenness.

Any concerns or questions about the plugin blocking lists should be directed to

Document Tags and Contributors

 Contributors to this page: bsmedberg
 Last updated by: bsmedberg,