Firefox 65 has re-added the support for the Internet Explorer-derived
Event.prototype.returnValue properties, which was once landed on Firefox 63 but soon disabled on non-Nightly channels due to several site compatibility issues.
In Firefox 65 and later, when the
keyCode property of the
keypress event is
0, the value will be the same as
charCode. Conversely, when
0, it will be the same as
keyCode. Although this mirroring behaviour matches other browsers and is expected to solve most of the compatibility issues, user agent sniffing might cause another issues like the Closure Library, which has already been solved by Google.
Web developers are also discouraged to use these once-non-standard properties for browser detection.