MouseEvent.offsetY properties have been standardized in the CSSOM View specification and now implemented in Firefox. This change exposes a bug in the Google Maps API’s Drawing Layer where incorrect mouse coordinates are returned likely due to a poor user-agent detection. Google is working on the fix.
Update: Google has not deployed the fix, but according to a comment in the bug, it is possible to workaround the issue by adding
v=3 to the Maps API script URL.
Update: The Maps API issue has been fixed by Google.
Update: Those properties have been disabled on mobile, both on Firefox for Android and Firefox OS, due to another issue on Google Maps regarding zooming, then enabled with Firefox 43. If you find any other issue, please let us know.