Firefox now supports the non-standard
srcElement property on the
Event interface for a better cross-browser compatibility. Firefox Nightly has already added the support since the version 60. It’s the standard
target property’s alias originated from Internet Explorer, which has been implemented in all other browsers by now.
Mozilla engineers are concerned about the possibility of broken sites if it’s misused for browser detection targeting Firefox. Make sure your site is not the case.
Update: Part of the content on QQ.com is not being displayed due to this change.