Lots to see in Firefox 93!

Firefox 93 comes with lots of lovely updates including AVIF image format support, filling of XFA-based forms in its PDF viewer and protection against insecure downloads by blocking downloads relying on insecure connections.

Web developers are now able to use static initialization blocks within JavaScript classes, and there are some Shadow DOM and Custom Elements updates. The SHA-256 algorithm is now supported for HTTP Authentication using digests. This allows much more secure authentication than previously available using the MD5 algorithm.

This blog post provides merely a set of highlights; for all the details, check out the following:

AVIF Image Support

The AV1 Image File Format (AVIF) is a powerful, open source, royalty-free file format. AVIF has the potential to become the “next big thing” for sharing images in web content. It offers state-of-the-art features and performance, without the encumbrance of complicated licensing and patent royalties that have hampered comparable alternatives.

It offers much better lossless compression compared to PNG or JPEG formats, with support for higher color depths and transparency. As support is not yet comprehensive, you should include fallbacks to formats with better browser support (i.e. using the <picture> element).

Read more about the AVIF image format in the Image file type and format guide on MDN.

Static initialization blocks

Support for static initialization blocks in JavaScript classes is now available in Firefox 93. This enables more flexibility as it allows developers to run blocks of code when initializing static fields. This is handy if you want to set multiple fields from a single value or evaluate statements.

You can have multiple static blocks within a class and they come with their own scope. As they are declared within a class, they have access to a class’s private fields. You can find more information about static initialization blocks on MDN.

Custom Elements & Shadow DOM

In Firefox 92 the Imperative Slotting API was implemented giving developers more control over assigning slots within a custom element. Firefox 93 included support for the slotchange event that fires when the nodes within a slot change.

Also implemented in Firefox 93 is the HTMLElement.attachInternals() method. This returns an instance of ElementInternals, allowing control over an HTML element’s internal features. The ElementInternals.shadowRoot property was also added, meaning developers can gain access to the shadow root of elements, even if they themselves didn’t create the element.

If you want to learn more about Custom Elements and the Shadow DOM, check out MDN’s guides on the topics.

Other highlights

A few other features worth noting include:

About Ruth John

Ruth John works as a Technical Writer at Mozilla. A recent addition to the MDN team, she's a big fan of web technologies, and not only enjoys writing about them, but building demos with them as well.

More articles by Ruth John…


  1. Thomas

    Is there a spelling mistake here, in “synthesis”?

    “supported for the font-sythesis property.”

    October 9th, 2021 at 00:32

    1. Ruth John

      Yes – thanks Thomas! Updated :)

      October 11th, 2021 at 08:19

  2. Abayomi Oyewumi

    Great to see firefox always pulling the strides.
    You guys deserve more clapps

    October 19th, 2021 at 14:37

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