The remote agent currently accepts an “address spec”:

% firefox --remote-debugger localhost:1234

It would be nice if we either changed the input to be a fully qualified URL
or allowed the scheme/path components to be optional:

% firefox --remote-debugger http://localhost:1234/foo/bar

This would let you bind the root of the HTTPD to a custom path name.

This would also allow the server to be served
on a different transport mechanism in the future, should we want to have that:

% firefox --remote-debugger tcp://localhost:9992/

Or:

% firefox --remote-debugger tcp+jsonrpc://localhost/

Or:

% firefox --remote-debugger tcp+cdpbinary://localhost/

As part of my review on https://phabricator.services.mozilla.com/D50289#inline-305770 I had the following comment, which needs to be further discussed on this bug:

Also "--remote-debugger" is not in alignment with the Chrome options, which isn't existent there but instead it has two individual entries for the address and port. Both prefixed with --remote-debugging-. Didn't we wanted to use the exact some options?

Another concern that came up in the security review was that the
transport mechanism isn’t encrypted. There are a lot of open questions
what it would mean to encrypt the HTTP transport that the WebDriver
WG needs to consider, but it is a matter that also affects how we
design the command-line flag. One could imagine something like this
to enable TLS:

% firefox --remote-debugger https://localhost:9992/

The --remote-debugger command line argument has been removed by bug 1669230.