Firefox 63 has implemented the
window.event property, which is derived from Internet Explorer but now standardized in the DOM spec along with
While this change aims at improving web compatibility, it’s expected that certain browser detection code using the once-non-standard property will rather be broken. A typical case already reported involves
0 in Firefox, which is being solved by Mozilla engineers. There might be other issues, so please scan your code and, if
window.event is found, make sure the functionality works with Firefox.
Note that the support for
srcElement has already been added with Firefox 62, and it also causes at least one compatibility issue.
Event.prototype.returnValue property has also been added with Firefox 63. The title of this post has been updated accordingly.
window.event is not going to ship with Firefox 63 due to several compatibility issues, mainly because Firefox doesn’t support the
keyCode property on it. Now
window.event is disabled on non-Nightly channels. The title of this post has been updated again.
Update 3: The support for
Event.returnValue has been temporarily disabled in Firefox 64 due to some broken web apps. It will be enabled again with Firefox 65 after the
keypress event’s compatibility issue is solved.