Windows – How to Set a Running Instance of Firefox as the “Default Browser”

Salvete! In Firefox we can create multiple profiles and have multiple instances of Firefox running, each with its own addons and customizations. So I have two profiles – one for browsing and one for development.

My problem is that after I have both profiles running, that I need to change which one is the "Default Browser". Whichever profile is launched first will be the "Default Browser" and the calls to url protocols (that is, external hyperlinks open will open in that particular instance).

This is not a matter of setting the default browser. If you go to Firefox's options and click the button to "Check Now" to see if it is the default browser, ANY instance of Firefox will know it is default. So "Default Browser" is not in relation to the "Default Instance".

This thread seems to throw a few clues:
http://www.toniblogs.com/04/2011/web/setting-default-firefox-profile/

And this one comes close, but neither does it have an answer: how to remotely open an URL in Firefox in a specific profile?

Also, in profiles.ini in the Firefox Application Data Folder, you can change the entry in th ini file to denote which instance is default:
Default=1
but it doesn't affect running instances, instead, it seems to only set the default selection in the profile switcher.

I know an alternative is to put one of the profiles in Firefox Portable, where, since it has a different application path, could be set as the "Default Browser" as opposed to an installed Firefox, but that is a workaround and doesn't answer my question.

My question therefore, is, how do I set one profile as default after it is already running? I don't want to restart all instances of Firefox to change the default. How can I do this midstream?

I suppose it could be possible either by a commandline action, or maybe there is a way to make Firefox reload this file? Some of the commenters mention some registry action, but does anyone have anything definite to try? Has anyone actually done this?