The field to enter the URL on Special:Interwiki is an <input type="url">, and Firefox checks that such fields contain valid URLs before allowing form submission. The protocol-relative syntax starting with //is treated as invalid - rightly, since it's not a real URL - so you can't submit the form. (Unless you use inspect element to change the input type to "text", which is what I did).
I think probably what should be done is just change the markup so the input type is "text".
Protocol-relative URLs should not generally be used anyways - if a site supports HTTPS, links to it should exclusively be HTTPS, and otherwise links will need to be HTTP. Protocol-relative URLs were only used on WMF wikis as a stopgap interim measure when secure viewing was being rolled out to everyone by default.
That makes good sense, but if the feature is still to be supported at all then it's probably worth the trivial fix to the markup, and if it should never be used then perhaps it should just be removed.