Firefox 90 for developers

• The response view now shows a preview for web fonts (Firefox bug 872078).

• A fix to the way that form payloads are handled around newline normalization and escaping in multipart/formdata. This meets the updated specification, and matches other browser implementations. (Firefox bug 1686765).
• Firefox now sets an image's intrinsic size and resolution based on EXIF information (if present and self-consistent). This allows a server to, for example, send a low-quality placeholder image to speed up loading. It also enables a number of other use cases (Firefox bug 1680387).

• -webkit-image-set() has been implemented as an alias of the standard image/image-set() function (Firefox bug 1709415).

• Private class static and instance fields and methods are now supported by default (Firefox bug 1708235 and Firefox bug 1708236).
• The in operator can now be used to check if a class private method or field has been defined. This offers a more compact approach for handling potentially undefined features, as oppose to wrapping code in try/catch blocks (Firefox bug 1648090).
• Custom date/time formats specified as options to the Intl.DateTimeFormat() constructor can now include dayPeriod — a value indicating that the approximate time of day (e.g. "in the morning", "at night", etc.) should be included as a narrow, short, or long string (Firefox bug 1645115).
• The relative indexing method at() has been added to the Array, String and TypedArray global objects. (Firefox bug 1681371)

• The HTTP fetch metadata request headers (Sec-Fetch-*) are now supported. These headers provide servers with additional context about requests, including whether they are same-origin, cross-origin, same-site, or user-initiated, and where/how the requested data is to be used. This allows servers to mitigate against several types of cross-origin attacks (Firefox bug 1695911).

Removals

• FTP has now been removed from Firefox (Firefox bug 1574475). This follows deprecation in Firefox 88. Note that web extensions can still register themselves as FTP protocol handlers.

DOM

• Support was added for the deprecated WheelEvent properties: WheelEvent.wheelDelta, WheelEvent.wheelDeltaX, and WheelEvent.wheelDeltaY. This allows Firefox to work with a small subset of pages that were broken by recent compatibility improvements to WheelEvent (Firefox bug 1708829).
• The CanvasRenderingContext2D interface of the Canvas API now provides a createConicGradient() method. This returns a CanvasGradient much like the existing linear and radial gradients, but allows a gradient to move around a point defined by coordinates. See Firefox bug 1627014 for more details.
• Support for the matrix protocol has been added and can now be passed into the Navigator.registerProtocolHandler() method as a valid scheme.

• Marionette now restricts to a single active WebDriver session (Firefox bug 1691047).
• Added support for the new type of user prompts in Firefox (Firefox bug 1686741)
• Window handles make use of a unique id now, and don't change for process swaps as caused by cross-group navigations (Firefox bug 1680479).
• Fixed an inappropriate abortion of the current WebDriver command when a new user prompt was opened in a background tab (Firefox bug 1701686).
• Fixed the WebDriver:GetWindowHandles command to now correctly handle unloaded tabs (Firefox bug 1682062).
• Fixed the WebDriver:NewSession command to always return the proxy capability even if empty (Firefox bug 1710935).

Removals

• With the removal of the FTP support in Firefox 90 the ftpProxy capability is no longer evaluated, and when used will throw an invalid argument error (Firefox bug 1703805).

• The matrix URI scheme is now supported and can be defined as a protocol within the protocol_handlers key in an extensions manifest.json
• Starting with this version, the Cache API can be used in the extension pages and worker globals. For more details, see (Firefox bug 1575625).