Firefox 91 release notes for developers

This article provides information about the changes in Firefox 91 that will affect developers. Firefox 91 was released on August 10, 2021.

[!NOTE] See also Hopping on Firefox 91 on Mozilla Hacks.

Changes for web developers

HTML

No changes

CSS

• A fix to how the {{cssxref("@counter-style/pad")}} descriptor handles the negative sign (Firefox bug 1714445).
• The -moz-tab-size property has been unprefixed to the standard {{cssxref("tab-size")}}, and the prefixed version maintained as an alias (Firefox bug 737785).

Removals

• The non-standard -moz-outline-radius property has been removed (Firefox bug 1715984). The property has not been usable by web developers since Firefox 88, this completes the removal.

JavaScript

• {{jsxref("Intl/DateTimeFormat/formatRange", "Intl.DateTimeFormat.prototype.formatRange()")}} and {{jsxref("Intl/DateTimeFormat/formatRangeToParts", "Intl.DateTimeFormat.prototype.formatRangeToParts()")}} are now supported in release builds. The formatRange() method returns a localized and formatted string for the range between two {{jsxref("Date")}} objects (e.g., "1/05/21 – 1/10/21"). The formatRangeToParts() method returns an array containing the locale-specific parts of a formatted date range (Firefox bug 1653024).
• The {{jsxref("Intl/DateTimeFormat/DateTimeFormat", "Intl.DateTimeFormat()")}} constructor allows four new timeZoneName options for formatting how the timezone is displayed. These include the localized GMT formats shortOffset and longOffset, and the generic non-location formats shortGeneric and longGeneric (Firefox bug 1653024).
• The {{jsxref("Global_Objects/Error/Error", "Error()")}} constructor can now take the error cause as value in the option parameter. This allows code to catch errors and throw new/modified versions that retain the original error and stack trace (Firefox bug 1679653).

HTTP

• The Gamepad API now requires a secure context (Firefox bug 1704005).

APIs

DOM

• The Visual Viewport API is now enabled by default on Firefox desktop releases (it has been enabled on Firefox for Android since version 68). The API provides access to information describing the position of the {{Glossary("visual viewport")}} relative to the document, as well as to the window's content area. It also provides events that allow changes to the viewport to be monitored. (Firefox bug 1551302).
• The Gamepad API is now protected by Feature-Policy: gamepad. If disallowed by the Permission Policy, calls to {{domxref('Navigator.getGamepads()')}} will throw a SecurityError {{domxref('DOMException')}}, and the {{domxref("Window.gamepadconnected_event", "gamepadconnected")}} and {{domxref("Window.gamepaddisconnected_event", "gamepaddisconnected")}} events will not fire. The default allowlist is *; this default will be updated to self in a future release, in order to match the specification. (Firefox bug 1704005).
• Window.clientInformation has been added as an alias for {{domxref("Window.navigator")}}, in order to match recent specification updates and improve compatibility with other major browsers (Firefox bug 1717072).
• Changing the playback speed of a media element (<video> or <audio>) using the playbackRate attribute now works when the media element is captured to a MediaStream or via AudioContext.createMediaElementSource (Firefox bug 1517199).

WebDriver conformance (Marionette)

• Fixed a bug, which caused the commands WebDriver:AcceptAlert and WebDriver:DismissAlert to hang for user prompts as opened in a popup window (Firefox bug 1721982).
• Fixed an inappropriate handling of the webSocketUrl capability, which would return true if webSocketUrl was not supported (Firefox bug 1713775).