Firefox 155 release notes for developers (Beta)

Changes for web developers

Developer Tools

• The media feature emulation buttons in the Rules view are now collected into a dedicated emulation panel, which is opened using the @ button. The panel also adds emulation of the prefers-reduced-motion media feature. (Firefox bug 1692434 and Firefox bug 1477920).
• The JSON Viewer now opens JSON Lines (NDJSON) documents, which are served as application/jsonlines, application/x-ndjson, or text/jsonl, or have a .jsonl file extension. Each line is parsed separately into its own collapsible entry, labeled with the line number it came from, and a line that fails to parse is reported inline without affecting the rest of the document. (Firefox bug 2055774, Firefox bug 2060972, and Firefox bug 2060529).
• Added a keyboard shortcut for disabling breakpoints in the Debugger. (Firefox bug 1642578).

CSS

• The attr() CSS function can now be used in any CSS property, rather than only in content. This lets you drive styling from HTML attributes, such as width: attr(data-size px), without using JavaScript. <attr-type> values (including unit identifiers such as px and s), fallback values, and namespaced attributes are now supported. You can now also use attr() inside container style queries. (Firefox bug 2038940).
• The progress() CSS function is now supported. This returns a <number> representing how far a value has progressed between a start and an end value. The result can then be used to calculate other values, for example opacity: calc(0.4 + progress(100cqw, 300px, 900px) * 0.6). (Firefox bug 2047345).
• The alpha() CSS function is now supported. It allows you to pass a color and get back the color with a different alpha (transparency) value, leaving other components of the color unchanged. Inside the function, you can use the alpha keyword to refer to the alpha channel of the original color, for example alpha(from var(--brand) / calc(alpha * 0.5)). (Firefox bug 2059738 and Firefox bug 2059988).
• The font-width CSS property is now supported, along with the font-width @font-face descriptor and the CSSStyleDeclaration.fontWidth property. This is the new name for the font-stretch property, which continues to work as a legacy alias. Note that computed style enumeration now returns font-width rather than font-stretch. (Firefox bug 1911075).

JavaScript

• The Promise.allKeyed() and Promise.allSettledKeyed() static methods are now supported, as defined in the TC39 await dictionary proposal. These behave like Promise.all() and Promise.allSettled(), respectively, except that they take an object of promises instead of an iterable. They fulfill with an object that has the same keys, so results can be read by name instead of by position. (Firefox bug 2057270).
• A module that fails to load because of a network error or an incorrect MIME type is no longer cached as a failure, so importing the same module specifier again can succeed once the server recovers. This applies to JavaScript, JSON, CSS, and text modules, loaded either statically or with dynamic import, in both windows and workers. Relatedly, <link rel="modulepreload"> now fires the load event rather than error for modules that are already fetched or still fetching, and a module script now loads even if an earlier modulepreload of the same URL failed its integrity check. (Firefox bug 2055211 and Firefox bug 2052949).

HTTP

• Firefox now uses Happy Eyeballs version 3 when establishing connections, racing IPv6 and IPv4 addresses so that connection setup is not delayed by an unreachable address family. Note that this is currently supported on some platforms only. (Firefox bug 2062892).
• QUIC version negotiation is now supported, allowing HTTP/3 connections to negotiate QUIC version 2. (Firefox bug 2059947).

APIs

• Several WebTransport API features are now supported:
  • Send groups let you group streams that should share bandwidth and prioritize streams relative to each other within a group. You can create one using WebTransport.createSendGroup(). Then pass the returned WebTransportSendGroup in the sendGroup option of WebTransport.createBidirectionalStream() or WebTransport.createUnidirectionalStream(). (Firefox bug 2007165).
  • The WebTransport.exportKeyingMaterial() method derives keying material from the underlying TLS connection for a given label and context, so that both endpoints can obtain the same shared secret. (Firefox bug 2007200).
  • The WebTransportDatagramDuplexStream.createWritable() method returns a WebTransportDatagramsWritable stream for sending datagrams, with sendGroup and sendOrder properties for prioritizing it against other senders. (Firefox bug 2007174).
  • The WebTransport() constructor accepts a protocols option listing the application protocols the client supports, which is sent to the server in the wt-available-protocols request header. The protocol chosen by the server is exposed in the WebTransport.protocol property. (Firefox bug 2007150).
  • The WebTransport.draining property indicates when the server has requested the client to start a graceful shutdown of the session. (Firefox bug 2007160).
• The WebGPU API now supports the dual-source-blending feature on desktop, which can be requested in GPUAdapter.requestDevice(). This allows src1, one-minus-src1, src1-alpha, and one-minus-src1-alpha to be specified in the srcFactor and dstFactor properties of createRenderPipeline() and createRenderPipelineAsync(). The WGSL dual_source_blending extension is also supported. (Firefox bug 1924328).

DOM

• The SVGAElement interface now implements the HyperlinkElementUtils mixin. As a result, SVG <a> elements expose the same URL component properties as HTML <a> elements: protocol, username, password, host, hostname, port, pathname, search, and hash. The read-only origin property is also exposed. (Firefox bug 2058578).
• The SVGNumberList, SVGPointList, SVGStringList, and SVGTransformList interfaces now support indexed setters. This means you can replace an item in the list using bracket notation, such as transformList[0] = newTransform, instead of calling replaceItem(). The SVGLengthList interface already supports indexed setters. (Firefox bug 2059426).
• The SVGGraphicsElement.getBBox() method now honors its options argument, with the fill, stroke, markers, and clipped properties. This allows you to get a bounding box that accounts for the stroke, markers, and clipping applied to an element, rather than only its fill geometry. (Firefox bug 2060873).
• Elements that are not rendered, such as those inside <mask>, <clipPath>, <marker>, <symbol>, and <defs>, now return an empty rectangle from Element.getBoundingClientRect() and an empty list from Element.getClientRects(), instead of reporting a box that was never painted. (Firefox bug 2061646).

Media, WebRTC, and Web Audio

• The error event fired on an RTCDataChannel object may now report sctp-failure in its error.errorDetail property if the transport is closed due to an error. In addition, RTCError and RTCErrorEvent are now available in dedicated workers (this exposure is not yet in the specification). (Firefox bug 1814460).
• The RTCPeerConnection.sctp property now returns an RTCSctpTransport at the times required by the specification, including in the have-remote-offer signaling state, where it was previously null. The transport now also reaches the connected and closed states, and its maxChannels and maxMessageSize properties are populated correctly. (Firefox bug 2019361 and Firefox bug 2056412).
• Two-byte RTP header extensions are now supported, so header extensions with an id of 15 or above can be negotiated instead of causing an OperationError. (Firefox bug 2014357).
• The selectedCandidatePairChanges property is now reported in RTCTransportStats. (Firefox bug 2055911).
• The transport statistics returned by RTCPeerConnection.getStats() are now correct before negotiation, in other words after setLocalDescription() but before a remote description has been set. The dtlsRole property is now reported as unknown until the DTLS handshake selects a role, where previously it was not reported at all (Firefox bug 2053296), and the iceState property now starts as new rather than checking, which incorrectly indicated that connectivity checks were already underway (Firefox bug 2053297).

WebAssembly

• The compact import section binary format extension is now supported, which reduces the size of modules that have many imports. (Firefox bug 2062344).
• The wide arithmetic proposal is now supported, adding the i64.add128, i64.sub128, i64.mul_wide_s, and i64.mul_wide_u instructions. These produce 128-bit results from 64-bit operands, which previously had to be emulated in code compiled to WebAssembly, such as bignum and cryptography libraries. (Firefox bug 2062374).

WebDriver conformance (WebDriver BiDi, Marionette)

General

• Disabled the download panel to prevent the current document from losing focus when a download begins. (Firefox bug 2035439).
• Fixed the Actions API so that the dblclick event is fired when performing a double-click while holding down the Ctrl key on non-macOS platforms. (Firefox bug 2058556).

WebDriver BiDi

• Updated the Mozilla-specific moz:debugging module to no longer rely on the same nested event loop API as DevTools, which prevents conflicts when WebDriver BiDi and DevTools are used in parallel. (Firefox bug 2041335).
• Fixed the browsingContext.reload command failing when used for frames. (Firefox bug 2030909).
• Removed support for the contexts argument in the session.unsubscribe command. From now on, clients can unsubscribe only by event name or subscription ID. (Firefox bug 1988723).

Changes for add-on developers

Experimental web features

These features are shipping in Firefox 155 but are disabled by default. To experiment with them, search for the appropriate preference on the about:config page and set it to true. You can find more such features on the Experimental features page.

• Scroll-driven animations: layout.css.scroll-driven-animations.enabled

  Scroll-driven animations let an animation progress with the scroll position of a scroller, or with the position of an element within its scroller, rather than with time. This preference covers the scroll-timeline and view-timeline properties and their longhands, including the view-timeline-inset property, along with the scroll() and view() functional notations. In this release the view-timeline-inset longhand was added to the view-timeline shorthand. (Firefox bug 2046602).

• CSS Typed Object Model Level 1: layout.css.typed-om.enabled

  The CSS Typed Object Model API exposes CSS values as typed JavaScript objects rather than strings, which simplifies manipulating CSS from script. (Firefox bug 1278697).

• at-rule() support queries: layout.css.supports.at-rule.enabled

  The at-rule() function in the @supports at-rule lets you test whether the browser supports a given CSS at-rule, for example @supports at-rule(@scope). (Firefox bug 2060754).

• Audio Session API: dom.audio_session.enabled

  The Audio Session API lets a site declare how its audio should behave relative to other audio playing on the device, such as whether it should mix with, duck, or interrupt other audio. (Firefox bug 2055710).

• CSS basic shapes allow farthest-corner and closest-corner keywords: layout.css.ellipse-corners.enabled

  The farthest-corner and closest-corner keywords can be used for specifying the radii values of the ellipse() and circle() CSS basic shapes. (Firefox bug 2037673).

• Truncating content with line-clamp: layout.css.line-clamp.enabled

  The line-clamp CSS property works without the -webkit- vendor prefix, and now also supports the no-ellipsis keyword and <string> values for choosing what is shown where the text is clamped. (Firefox bug 2042999 and Firefox bug 2043000).

• Scoped custom element registries: dom.scoped-custom-element-registries.enabled

  A CustomElementRegistry can be constructed and passed to Element.attachShadow(), so that a shadow root can define custom elements that do not clash with those defined in the global registry. (Firefox bug 2018900). This release also adds the customelementregistry global attribute, for selecting the registry an element is associated with from markup. (Firefox bug 2029965).

• Buffer boundary assertions in regular expressions: (Nightly only) javascript.options.experimental.regexp_buffer_boundaries

  The TC39 RegExp buffer boundaries proposal adds the \A, \z, and \Z assertions to regular expressions. These match the start or end of the entire input regardless of whether the m flag is set. (Firefox bug 2047706).

• border-area value for background-clip: layout.css.background-clip.border-area.enabled

  The border-area value of the background-clip CSS property clips the background to the area painted by the element's border, which makes it possible to use a gradient or image as a border. (Firefox bug 2045230).