Firefox 65 for developers

This article provides information about the changes in Firefox 65 that will affect developers. Firefox 65 was released on January 29, 2019.

Changes for web developers

Developer tools

  • The Flexbox inspector is now enabled by default.
  • Support has been added to the JavaScript Debugger for XHR Breakpoints (Firefox bug 821610).
  • Right-click on an item in the accessibility tree from the Accessibility viewer to print it as JSON to the JSON viewer.
  • The color contrast display of the Accessibility Picker has been updated so that if a text's background is complex (e.g. a gradient or complex image), it shows a range of color contrast values.
  • The Headers tab of the Network Monitor now displays the Referrer Policy for the selected request (Firefox bug 1496742).
  • When displaying stack traces (e.g. in console logs or the JavaScript debugger), calls to framework methods are identified and collapsed by default, making it easier to home in on your code.
  • In the same fashion as native terminals, you can now use reverse search to find entries in your JavaScript console history (F9 on Windows/Linux or Ctrl + R on macOS, then type a search term, followed by Ctrl + R/Ctrl + S to toggle through results).
  • The JavaScript console's $0 shortcut (references the currently inspected element on the page) now has autocomplete available, so for example you could type $0.te to get autocomplete suggestions for properties like $0.textContent.
  • The edits you make in the Rules view of the Inspector are now listed in the Changes panel (Firefox bug 1503920).



  • The image-rendering property's crisp-edges value has now been unprefixed (Firefox bug 1496617).
  • A scrollbar-color value of auto now resolves to auto, rather than two colors (Firefox bug 1501418).
  • The break-* properties have been implemented, and the legacy page-break-* properties have been aliased to them (Firefox bug 775618):
  • The overflow-wrap property's anywhere value has been implemented (Firefox bug 1505786).
  • The new step position keywords jump-start, jump-end, jump-none, and jump-both — usable inside the steps() timing function — have been implemented (Firefox bug 1496619). This also coincides with the removal of the frames() timing function, which was the previous way of implementing such functionality, now deprecated.
  • Some new -webkit-appearance values have been added, for compatibility with other browsers. In particular:
    • meter, which is now used as the default value for <meter> elements in UA stylesheets. The existing value meterbar is now an alias for meter (Firefox bug 1501483).
    • progress-bar, which is now used as the default value for <progress> elements in UA stylesheets. The existing value progressbar is now an alias for progress-bar (Firefox bug 1501506).
    • textarea, which is now used as the default value for <textarea> elements in UA stylesheets. The existing value textfield-multiline is now an alias for textarea (Firefox bug 1507905).
  • The behavior of user-select has been changed to make it align more with other browsers (Firefox bug 1506547). Specifically:
    • user-select: all set on an element no longer overrides other values of user-select set on children of that element. So for example in the following snippet:
      <div style="-webkit-user-select: all">
        <div style="-webkit-user-select: none">None</div>
      The <div> with none set on it is now non-selectable. Previously this value would have been overridden by the all value set on the parent element.
    • non-contenteditable elements nested inside contenteditable elements are now selectable.
    • user-select now behaves consistently inside and outside shadow DOM.
    • The proprietary -moz-text value has been removed.
  • CSS environment variables (the env() function) have been implemented (Firefox bug 1462233).



No changes.



New APIs


DOM events

Web workers

Fetch and Service workers

Media, Web Audio, and WebRTC

Canvas and WebGL


  • Mutation events have been disabled in shadow trees (Firefox bug 1489858).
  • The non-standard MediaStream property currentTime has been removed (Firefox bug 1502927).
  • The dom.webcomponents.shadowdom.enabled and dom.webcomponents.customelements.enabled prefs have been removed — Shadow DOM and Custom Elements can no longer be disabled in about:config (Firefox bug 1503019).
  • The non-standard DOM text event — fired to notify the browser editor UI of IME composition string data and selection range — has been removed (Firefox bug 1288640).
  • The keypress event is no longer fired for non-printable keys (Firefox bug 968056), except for the Enter key, and the Shift + Enter and Ctrl + Enter key combinations (these were kept for cross-browser compatibility purposes).



No changes.


No changes.

WebDriver conformance (Marionette)

API changes

  • WebDriver:ElementSendKeys now handles <input type=file> more relaxed for interactability checks, and allows those elements to be hidden without raising a not interactable error anymore. If a strict interactability check is wanted the capability strictFileInteractability can be used (Firefox bug 1502864).

Bug fixes

  • The window manipulation commands WebDriver:FullscreenWindow, WebDriver:MinimizeWindow, WebDriver:MaximizeWindow, and WebDriver:SetWindowRect have been made more stable (Firefox bug 1492499). It means that under special conditions they don't cause an infinite hang anymore, but instead timeout after 5s if the requested window state cannot be reached (Firefox bug 1521527).
  • WebDriver:ElementClick now correctly calculates the center point of the element to click, which allows interactions with dimensions of 1x1 pixels (Firefox bug 1499360).


  • For unexpected alert open errors more informative messages are provided (Firefox bug 1502268).


  • Support for WebP images has been added (Firefox bug 1294490).
    • In addition, to facilitate cross-browser compatibility in certain situations the WebP MIMEType (image/webp) has been added to the standard HTTP Request Accept header for HTML files (Firefox bug 1507691).
  • The AV1 codec is now supported by default on Windows (Firefox bug 1452146).

Changes for add-on developers

API changes


  • The tabs API has been enhanced to support tab successors — a tab can have a successor assigned to it, which is the ID of the tab that will be active once it is closed (Firefox bug 1500479, also see this blog post for more information). In particular:
    • The tabs.Tab type now has a successorId property, which can be used to store/retrieve the ID of the tab's successor.
    • The tabs.onActivated event listener's callback has a new parameter available, previousTabId, which contains the ID of the previous activated tab, if it is still open.
    • The tabs.update() function's updateProperties object has a new optional property available on it, successorTabId, so can be used to update it.
    • successorTabId is also returned by functions like tabs.get() and tabs.query().
    • The new function tabs.moveInSuccession() allows manipulation of tab successors in bulk.

Manifest changes

No changes.


See also

Older versions