Good luck but I am not hopeful Firefox's behavior will be corrected. See also this 13 year old pearl courtesy of Bugzilla:

"Set screen coordinates during HTML5 drag event"

> The current HTML5 spec describes that all DragEvent properties should be available during all the events - according to editor Ian Hickson.

>> Note though that it doesn't specify what the properties should be set to, just that they should be set and we currently set them to 0.

https://bugzilla.mozilla.org/show_bug.cgi?id=505521