What steps will reproduce the problem?
1. Firefox supports the Registrar Protocol methods as described by HTML5
And
1. Currently - No support for URL loading?!? :-(

It would be nice to supply a file on the URL for KitCAD i3 KGC to open for 
editing.

EG: Index.html?url=http%3F%2F%2Fexample.com%
2Fsomefile.kp2#81519201192038514161751931513-72151920

Only down side is the automated deleting of current uploaded file (may 
require a prompt or a file manager for KitCAD i3 save files).