Firefox 90 for developers

This article provides information about the changes in Firefox 90 that will affect developers. Firefox 90 was released on July 13th, 2021.

Note: See also Getting lively with Firefox 90 on Mozilla Hacks.

Changes for web developers

Developer Tools


  • 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).




  • 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).




WebDriver conformance (Marionette)


Changes for add-on developers

Older versions