Firefox 121 for developers

• Lazy loading of <iframes> is now supported, allowing developers to hint that particular <iframe>s should only be loaded when (and if) they become visible. This can speed up initial load time by reducing the resources that need to be fetched on page load (some <iframes> may not need to be fetched at all). The hint can be provided via the loading attribute on the <iframe> element, or in JavaScript using HTMLIFrameElement.loading. (Firefox bug 1622090).

Removals

• The :has() selector, commonly referred to as the parent selector, is now supported. It allows developers to apply styles to an element based on related elements, e.g. li:has(ul) will match a list which contains a sub-list, or h1:has(+ p) will match a heading with a paragraph directly after it.
• The text-indent CSS property now supports the each-line and hanging property values (Firefox bug 784648), allowing developers to more easily specify certain text indentation styles. Additionally, developers can now combine multiple text-indent property values for more creativity, e.g. text-indent: 3em hanging each-line.

Removals

• Date.parse() now accepts several additional date formats:
  • Year > 9999 for YYYY-MMM-DD format (e.g. 19999-Jan-01) (Firefox bug 1858851)
  • MMM-DD-YYYY (e.g. Jan-01-1970) (Firefox bug 1863489)
  • Milliseconds for non-ISO date formats (e.g. Jan 1 1970 10:00:00.050) (Firefox bug 1863125)
  • Day of week at the beginning of formats which were being rejected, such as:
    • Wed, 1970-01-01
    • Wed, 1970-Jan-01
    The day of week does not need to be correct, or a day of week at all; for example, foo 1970-01-01 works (Firefox bug 1617562).
• Other Date.parse() fixes:
  • YYYY-M-DD and YYYY-MM-D are no longer assumed GMT as an ISO date YYYY-MM-DD would be (Firefox bug 1783731).
  • Milliseconds for all formats are truncated after 3 digits, rather than being rounded (Firefox bug 746529).

Removals

Removals

Removals

Removals

• The sendOrder property of the WebTransportSendStream interface is now supported, allowing the relative priority of outgoing WebTransport API bidirectional streams to be read and changed after the stream is created. Note that this cannot be used to modify the priority of unidirectional streams, as Firefox implements them as WritableStream instead of WebTransportSendStream. (For more information see Firefox bug 1853444.)

DOM

Media, WebRTC, and Web Audio

Removals

Removals

General

WebDriver BiDi

Marionette

• Fixed a bug that resulted in background pages not starting when a manifest.json background key contains service_worker and scripts declarations (Firefox bug 1860304).

  Note: Incidentally, a change in Chrome 121 sees the scripts property ignored when specified with the service_worker property. Previously, Chrome refused to load extensions containing both properties (Chromium bug 1418934). For more information, see Browser support of the background manifest key.