About this project

What is Firefox site compatibility? Why it's so important? Here are the answers.

Who we are

The Firefox Site Compatibility Working Group is an independent community initiative that tackles the backward compatibility and regressions of Firefox through monitoring sources, providing documents and tools, and promoting engagement with Web developers. While Mozilla’s Compatibility team is doing fantastic jobs in improving cross-browser compatibility, there are no staffers intensively covering compatibility issues in Firefox itself, so we are here to serve voluntarily. Our missions are to:

  • Improve Firefox user experience by preventing site breakages
  • Engage more with Web developers through various channels
  • Make Firefox a leading brand and robust Web platform for the millions of users

Why important

The world of the Web is rapidly evolving in this HTML5 era. Mozilla is a pioneer in rich, modern Web technologies, and Firefox is trying to keep up with the changes made to standard specifications such as HTML, CSS, DOM and ECMAScript. As part of the standardization efforts, Firefox is also dropping a bunch of the legacy, non-standard features originated from the Netscape browser. Those changes that we are covering here are necessary for a better Web.

It’s obviously impossible for Mozilla to test Firefox with all the sites around the world, and that’s why Firefox releases sometimes contain regressions that may break your website or application. For whatever reason, site breakages lead to customer defection from Firefox; therefore, we believe that solving regressions and communicating changes in advance are crucial to the success of the Firefox ecosystem. With the help of Web developers like you, most of Firefox regressions can hopefully be found and fixed during the alpha or beta cycle.

What we do

We have two focus areas:

  • Backward compatibility: While Mozilla is baking early tester builds called Firefox Nightly, which comes before Firefox Developer Edition, we post a series of site compatibility documents to let Web developers know backward-incompatible changes in that release. At that point, you should have at least 8 weeks to make necessary changes to your code before it goes public.
  • Regressions: Every 4 weeks, after a final version of Firefox hits the streets, we spend time finding regressions by monitoring various channels, including Firefox Support Forum, Bugzilla, Stack Overflow and Reddit. If a potential issue is found, we analyze it, report it to Bugzilla if not filed yet, then encourage Firefox developers to fix it. Web developers can be notified of the issue with a workaround, if any, by following us on Twitter.

While we may cover cross-browser compatibility or interoperability on our blog, those topics are not our primary focus. Mozilla’s Compatibility team is doing excellent work in that field, so it’s recommended to follow their activities as well.

Who we exactly are

This initiative is led by Kohei Yoshino, a long-time Mozilla contributor. He has been an active Mozilla community member since 2002 and also worked for now-gone Mozilla Japan Ltd. as a full-time employee from 2004 to 2012. Actually, our older documents were first written in Japanese as an internal project of Mozilla Japan Ltd.

Help is always welcome! See the Contribute page for what you can do. If you have any questions or feedback, feel free to reach us out at any time via one of our social media or email.


While this project remains independent, we have been partially backed by Mozilla. As mentioned above, our older documents were created in-house at now-gone Mozilla Japan Ltd. From January to June 2018, our documentation work was once again funded by the Mozilla Corporation.

We also thank individuals who have supported our initiative by tipping generously or giving feedback!