Ability to start Firefox with remote debugging enabled

[sole]

Currently to enable remote debugging you have to open the developer console and type listen {port_number}

This makes it impossible to run functional tests as we cannot launch Firefox and have it run on an specific port.

The solution would be to be able to:

firefox --remote-debugging-port=6000

there was some work going on this already: https://bugzilla.mozilla.org/show_bug.cgi?id=942756

[jryans]

It's sad that bug 942756 never landed, but it's a large patch set that I feel like is unlikely to revive.

I filed another bug to do only what's required here.

[jryans]

The new bug landed "--start-debugger-server" in Firefox 37.