Firefox 74 for developers

This article provides information about the changes in Firefox 74 that will affect developers. Firefox 74 was released on March 10, 2020.

Changes for web developers

Developer tools

Web console


No changes.




No changes.


  • The Optional chaining operator has been implemented (Firefox bug 1566143).
  • When a JavaScript URL (javascript:) is evaluated and the result is a string, this string is parsed to create an HTML document, which is then presented. Previously, this document's URL (as reported by the document.location property, for example) was the originating javascript: URL; it is now correctly the URL of the document the javascript: URL was evaluated in (Firefox bug 836567).


  • The Object.toSource() method and the global function uneval() are no longer available for use by web content or extensions (bug 1565170).



DOM events

  • Firefox 74 now supports the languagechange_event event and its companion event handler property, onlanguagechange, which is triggered when the user changes their preferred language (Firefox bug 1154779). This was previously listed in our compatibility database as supported from Firefox 3.5, but this was in error.

Canvas and WebGL




  • TLS 1.0 and 1.1 support has been removed from Firefox; you'll need to make sure your web server supports TLS 1.2 or 1.3 going forward. From now on, Firefox will return a Secure Connection Failed error when connecting to servers using the older TLS versions (Firefox bug 1606734).
  • Starting in Firefox 74, when a site delegates permission to access a resource to embedded content in an <iframe> using the allow attribute, and the embedded page requests permission to use that resource, the parent page prompts the user for permission to use the resource and share it with the embedded domain, rather than both the outer and inner pages prompting the user for permission. If the outer page doesn't have the permission requested by the allow attribute, the <iframe> is immediately denied access without prompting the user Firefox bug 1483631.


No changes.

WebDriver conformance (Marionette)

  • Added WebDriver:Print to print the current page as a PDF document (Firefox bug 1604506).
  • Webdriver:TakeScreenshot now always captures the top-level browsing context and not the currently-selected browsing context, if no element to capture has been specified (Firefox bug 1398087, Firefox bug 1606794).
  • Using Webdriver:TakeScreenshot's full argument causes the complete page to be captured (Firefox bug 1571424).

Changes for add-on developers

API changes

Manifest changes

No changes.

See also

Older versions