In our Developer Edition and Nightly release channels, Firefox engineers help to test web standards addressing both developers' routine needs and the latest cutting-edge tech. They also participate in discussions that contribute to standards development.

The goal of this project is to ensure that everyone has access to web content and to web compatibility bug information. It was developed by volunteers and is supported by Mozilla. For more information about the vision of, how to submit bugs, and the bug lifecycle, visit the About page.

When bugs result in Firefox patches, they'll be listed in your Firefox install at about:compat.


If you have found a compatibility bug in a pre-release edition of Firefox (Beta, Developer Edition, and Nightly) for desktop or mobile, you can submit it via the "Report Site Issue" menu in the page actions menu


If you have found a compatibility bug in release Firefox, submit it at, or install and use our web compatibility reporter add-on.

Report a compatibility bug


This page lists patches enabled to support web content compatibility in Firefox. There are two types of patches: user agent overrides and interventions.

User agent overrides

Websites identify your browser with its User-Agent string. Sites configured to reject a browser will only send limited content to it, or none at all. The workaround for this is masking the User-Agents of rejected browsers with overrides.


Where User-Agent masking is insufficient or irrelevant, Firefox can be patched to make code modifications while rendering.


To access this page, visit the address about:compat with your browser (no leading https://).


The Firefox Site Compatibility Working Group

This volunteer initiative provides detailed notes about compatibility support in each version of Firefox, from 16 onwards.

View the notes