Custom protocol handler does not get arguments

Dear community, I am trying to handle custom protocol with Firefox, but Firefox does behave as it should. I would like to open the protocol "foo" with a custom application. - In "about:config", I have added the following key: network.protocol-handler.expose.foo - I set its value to false. - I made a link on a webpage pointing to 'foo:bar'. - I clicked on the link. - I chose my application, which just displays the argument list. - The argument list is always empty (was expecting 'bar' or 'foo:bar' or something). - Instead of my application, I tried to use the mail client (for debugging purpose, ultimately I want my application to be run). - I clicked on the link. - The mail client run, but I get an error: "The operation couldn’t be completed. (MCMailErrorDomain error 1030.). No associated application could be found". - I changed the link to 'mailto:bar'. - I clicked on the link. - The mail client open normally and create an email with 'bar' as recipient. It seems Firefox is not handling custom protocol the same way it handles 'mailto'. What can I do to solve this issue?