Under some circumstances (at least in Nublado) if display_url() is called on the extracted firefly_client object, it returns the wrong URL, omitting the "slate" endpoint.

Merlin Fisher-Levine is using this function to work around DM-29653, which should be fixed, but this problem obviously also should be fixed.

The use of display_url() can be helpful in some legitimate circumstances, so it should be properly documented, as well as work correctly.