DOM events changes introduced in Firefox 66

Firefox 66 introduced the following DOM events changes for compatibility with legacy content:

  • The Window.event and Event.returnValue properties — originally proprietary Internet Explorer features, then also supported across other browsers for compatibility purposes — have been added in Firefox 66.
  • From Firefox 66 onwards, the KeyboardEvent.keyCode property of the keypress event object will be the same as KeyboardEvent.charCode . This mirroring behavior matches other browsers and is expected to solve most associated compatibility issues, however user-agent sniffing might cause further issues in some JavaScript libraries.

If you maintain legacy intranet or hosted software that depends on Firefox not supporting these, the following preferences exist to opt into the pre-Firefox 66 behavior:

  • dom.keyboardevent.keypress.hack.dispatch_non_printable_keys.addl
  • dom.keyboardevent.keypress.hack.use_legacy_keycode_and_charcode.addl

These preferences take a comma-delimited list of domains as a value. For example, or

These preferences can be set via the new Preferences enterprise policy.

Support for window.event can be disabled by setting dom.window.event.enabled to false.

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