Firefox 70 for developers

This article provides information about the changes in Firefox 70 that will affect developers. Firefox 70 is the current Beta version of Firefox, and will ship on October 22, 2019.

Changes for web developers

Developer tools

Debugger updates

  • In the Debugger you can now set breakpoints for DOM Mutation, so execution will pause when a node or its attibutes are changed or when a node is removed from the DOM (bug 1576219).
  • The Debugger now shows an overlay on the page when it is paused, with basic stepping buttons to let you step and continue (bug 1574646).
  • The Debugger now shows sources that already got discarded by the engine (usually scripts that execute once during page load), so you can properly set breakpoints to debug when they execute next (bug 1572280).
  • The Debugger's scopes panel grouping has been simplified, consolidating additional scopes previously shown above the top level function (e.g. blocks created by let, with, or if/else) (bug 1448166)
  • The Debugger now retains the currently selected and expanded variables in the scopes panel while stepping (bug 1405402).
  • The Debugger now handles stepping over async functions correctly, making asynchronous function debugging easier (bug 1570178).
  • When debugging in Container sessions (useful for testing different logins), the sources in the debugger are now shown correctly (bug 1375036).
  • debugger statements can be now disabled in the Debugger by setting a breakpoint on them and switching the breakpoints to "Never pause here" (bug 925269).

Other updates


  • Firefox can now suggest securely-generated passwords to the user in the following situations:
    • An <input> element has the autocomplete="new-password" attribute value.
    • The user opens the context menu on any password input element, even if it is not intended for new passwords.




  • Cut, copy, and paste events are now dispatched to SVG graphics elements (bug 1569474).




Workers/Service workers

Media, Web Audio, and WebRTC

Canvas and WebGL


  • The default referrer policy for third-party tracking resources is now strict-origin-when-cross-origin when Enhanced Tracking Protection is turned on (bug 1569996).
  • The size of the Referer request header is now limited to 4 KB (4,096 bytes). If an overly long referer exceeds the defined limit, only the origin part will be sent (bug 1557346).
  • The HTTP cache is now partitioned per the top-level document's origin (bug 1536058).


Changes for add-on developers

API changes

  • Added a new parameter to the topSites.get() method that causes the method to return the list of pages that appear when the user opens a new tab (bug 1568617).

Manifest changes


The following theme key properties, which provided aliases for theme keys used in chromium-based browsers, were removed:

  • images property headerURL, themes should now use theme_frame.
  • colors properties:
    • accentcolor, themes should now use frame.
    • textcolor, themes should now use tab_background_text.

See also

Older versions