Firefox 128 for developers

• The target attribute of the <base> element now disallows ASCII newlines, tabs, or the < character, changing the value to _blank if any are present. This prevents dangling markup injection attacks that use an unclosed target attribute (Firefox bug 1835157).

• Relative color syntax is now enabled by default. Relative color syntax allows you to create a color value relative to an origin color, and can allow you to change a color in a different color space using color functions (Firefox bug 1900251).
• The content property now supports alternative text for content that includes an image. The alternative text is then exposed to the browsers accessibility tree. (See Firefox bug 1281158 and Firefox bug 1896047).

Removals

• The masonry layout properties align-tracks and justify-tracks have been removed. These properties were only implemented in Firefox and were recently dropped from the spec (Firefox bug 1900195).

• Resizeable ArrayBuffer and growable SharedArrayBuffer are now supported, allowing the size of buffers to be changed without having to allocate a new buffer and copy data into it (Firefox bug 1884150). The relevant methods and properties are:
  • Grow SharedArrayBuffer using the SharedArrayBuffer.prototype.grow() method. The maximum allowed size of the buffer is specified using the options.maxByteLength parameter to the SharedArrayBuffer() constructor. The SharedArrayBuffer.prototype.growable and SharedArrayBuffer.prototype.maxByteLength properties indicate whether the buffer can grow, and its maximum allowed size, respectively.
  • Resize ArrayBuffer using the ArrayBuffer.prototype.resize() method. The maximum allowed size of the buffer is specified using the options.maxByteLength parameter to the ArrayBuffer() constructor. The ArrayBuffer.prototype.resizable and ArrayBuffer.prototype.maxByteLength properties indicate whether the buffer can be resized, and its maximum allowed size, respectively.

• The HTTP Accept header in default requests and image requests now includes the image/svg+xml MIME type (Firefox bug 1711622).
• The RFC 9218: Extensible Prioritization Scheme for HTTP is now supported, including the HTTP Priority request and response header, which allows clients to hint at the expected relative priority for resources sent over a connection, and the HTTP/2 and HTTP/3 PRIORITY_UPDATE frames that allow the priority to be subsequently changed after the header has been sent (Firefox bug 1865040).

• RTCRtpReceiver.getParameters() and RTCRtpSender.getParameters() are now supported, returning an object that describes the current codecs used for the encoding and transmission of media on the receiver and sender tracks, respectively. (Firefox bug 1534687).
• Request.bytes() and Response.bytes() are now supported as a convenient way to get a Uint8Array from a Request and Response, respectively. (Firefox bug 1896475).
• PushMessageData.bytes() is now supported for returning the data from a push message as an array of bytes in a Uint8Array object. (Firefox bug 1897871).
• Blob.bytes() is supported for returning the data from a Blob as an array of bytes in a Uint8Array object. (Firefox bug 1896509).
• MediaKeys.getStatusForPolicy() is now supported for checking whether the CDM module, which is used to decrypt DRM protected content, would allow the presentation of encrypted media data for a "hypothetical" key based on specified policy requirements such as the High-bandwidth Digital Content Protection (HDCP) version supported by the system. This provides an application with a simple mechanism to know in advance whether playback at the optimal resolution will be allowed, without having to create a media key session or fetch a real license. (Firefox bug 1878714).
• RTCRtpTransceiver.setCodecPreferences() is now supported for setting the codecs that a WebRTC local peer is able to use for decoding received data, in its preferred codec order. Web applications can use this to cause the remote peer to choose a preferred codec, and to disable the negotiation of specific codecs — including those used for retransmission, redundancy, and forward error correction. (Firefox bug 1396922).
• Serialization of declarative shadow DOM, including the methods ShadowRoot.getHTML() and Element.getHTML(), and associated properties ShadowRoot.serializable and HTMLTemplateElement.shadowRootSerializable.
• The CSSPropertyRule interface is now supported by default and represents a CSS @property at-rule. The interface allows you to get the values, including name, syntax, inherits, and initialValue, of CSS custom properties defined using the @property at-rule (Firefox bug 1864818).
• The registerProperty() method is now supported by default. It allows you to define CSS custom properties via JavaScript, which is similar to using the @property at-rule in CSS (Firefox bug 1864818).

Media, WebRTC, and Web Audio

Removals

• The non-standard HTMLMediaElement.seekToNextFrame() method has been removed, and is now not supported by any browser. (Firefox bug 1336404).

General

• We now support the extended "unhandledPromptBehavior" capability which can either be a string (WebDriver classic) or a JSON object (WebDriver BiDi). The object type offers more capabilities for WebDriver BiDi like handling "beforeunload" prompts. (Firefox bug 1884650)

WebDriver BiDi

• Added support for the "BiDi flag" of a WebDriver Session to align with the WebDriver BiDi specification. This allows to identify sessions created for or upgraded to WebDriver BiDi. (Firefox bug 1898719)
• Added support for several arguments for the network.continueRequest command, which now allows to modify headers, cookies, method and body of a request before it is sent over the network. (Firefox bug 1850680)
• Added support for the userContext argument in the permissions.setPermission command, which allows to isolate a permission to a specific user context (implemented as containers in Firefox). (Firefox bug 1894217)
• Fixed a bug in browsingContext.navigate where a navigation error would load an error page and cause subsequent commands to fail. (Firefox bug 1878690)
• We fixed the order in which network.responseCompleted events are emitted for redirects. The original request's responseCompleted is now always emitted before the events for the redirect. (Firefox bug 1879580)
• To align with the current Firefox behavior, we introduced the workaround to not partition cookies which are added with the "storage.setCookie" command for the same domain as the page loaded in the targeted context. (Firefox bug 1898222)
• The input.setFiles command has been updated to throw an UnsupportedOperation error if the specified file does not exist. (Firefox bug 1887644)

Marionette

• Added support for the "HTTP flag" of a WebDriver Session to align with the WebDriver classic specification. This allows to identify sessions created for WebDriver classic. (Firefox bug 1884090)
• Added support for the Permissions API in WebDriver Classic. (Firefox bug 1524074)

• Adds the ability to enable and disable rules in static declarative net request rulesets with declarativeNetRequest.updateStaticRules and list disabled rules for a static ruleset with declarativeNetRequest.getDisabledRuleIds (Firefox bug 1810762).
• A static declarative net request rule, one defined through the declarative_net_request manifest key, is now loaded when it contains unrecognized properties but is otherwise valid (Firefox bug 1886608).
• Introduces MAX_NUMBER_OF_DYNAMIC_RULES and MAX_NUMBER_OF_SESSION_RULES to declarativeNetRequest. These properties represent the maximum number of dynamic and session-scoped rules an extension can add. They replace MAX_NUMBER_OF_DYNAMIC_AND_SESSION_RULES, which is now deprecated (Firefox bug 1894128).
• The default value of proxy.settings property proxyDNS is now false when using SOCKS4 and true when using SOCKS5. Previously, it defaulted to false for SOCKS4 and SOCKS5 (Firefox bug 1741375).
• Support is now provided for webRequest.onAuthRequired to handle authentication requests asynchronously by specifying "asyncBlocking" in the addListener parameter extraInfoSpec (Firefox bug 1889897).
• The optional_host_permissions manifest key has been added. This key enables runtime requests for access (access granted by the user after an extension has been installed) for the APIs in the extension that read or modify host data (Firefox bug 1766026).
• The non-standard Web API events overflow and underflow have been deprecated. Use of these events should be removed from extension documents before the release of Firefox 131 (Firefox bug 1898445).
• Support is now provided for scripts to run in the web page execution environment. This is provided through support for MAIN in ExecutionWorld for the scripting API, the addition of world to the contentScripts.register() API, and support for world in the content_scripts manifest key (Firefox bug 1736575).
• The scripting API can now inject scripts and CSS into sandboxed pages with about:blank, about:srcdoc, and data: URLs. This was implemented for scripting.executeScript, scripting.insertCSS, and scripting.removeCSS in Firefox bug 1475831 and scripting.registerContentScripts and scripting.updateContentScripts in Firefox bug 1853411 through the introduction of matchOriginAsFallback to scripting.RegisteredContentScript.
• Content scripts now run on sandboxed http, https, and file: URLs (Firefox bug 1411641).
• The manifest key content_scripts now supports match_origin_as_fallback and contentScripts.register matchOriginAsFallback, enabling scripts to be injected into about:, data:, and blob: pages when the document origin is opaque due to the use of CSP or iframe sandbox (Firefox bug 1475831 and Firefox bug 1896669). In addition, scripts registered with the content_scripts manifest key can now only run in blob: pages when match_origin_as_fallback is true (Firefox bug 1897113).
• Support added for the declarativeNetRequest.RuleCondition property domainType (Firefox bug 1797408).
• Extensions containing an unrecognized property in manifest key browser_specific_settings.gecko now load with a warning. Previously, these extensions errored on installation. This ensures that if a new browser_specific_settings.gecko property is added, extensions using that new property will load in versions of Firefox back to this release (Firefox bug 1757293).

These features are newly shipped in Firefox 128 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.

• image/jxl MIME type in Accept header for default and image requests: image.jxl.enabled. The HTTP Accept header in default requests and image requests can be configured to indicate support for the image/jxl MIME type. (Firefox bug 1711622).
• Cookies Having Independent Partitioned State (CHIPS): network.cookie.CHIPS.enabled. CHIPS, or "partitioned cookies", allow developers to opt a cookie into partitioned storage using the partitioned directive of the Set-Cookie HTTP header. When set, cookies have separate storage for each top-level site, and can only be read within the same top-level site they were set on and its subdomains. This blocks cross-site tracking, while still enabling legitimate uses of third-party cookies such as persisting state of embedded maps or chat widgets across different subdomains of a site. (Firefox bug 1898253).
• Privacy Preserving Attribution API (PPA): dom.origin-trials.private-attribution.state. PPA API provides an alternative to user tracking for ad attribution using the new navigator.privateAttribution object with saveImpression() and measureConversion() methods. Read more about PPA in the explainer. This experiment can be enabled for websites via origin trial or in the browser by setting the preference to 1. (Firefox bug 1900929).