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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?

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If I recall correctly, popping in the profile switch on the default browser registry entry should force Firefox to open the URL with a certain profile. –  digitxp Jul 27 '12 at 17:10
What do you mean? If I open the profile-switcher again and close it, it doesn't make any difference. –  BGM Jul 27 '12 at 18:52
What operating system are you using? If it's *nix you can probably adjust /usr/share/applications/defaults.list to point to a firefox.desktop link that points to the profile you want. –  Brandon Bertelsen Aug 2 '12 at 6:32
Window-7 - I added a tag for my post. –  BGM Aug 3 '12 at 2:01
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2 Answers

That would depend on the OS and how preferences are loaded for associations to http://, url and .htm/.html shortcut suffixes at a minimum. There are other associations to applications which also affect default browser such as opening within a MIcrosoft Application and mailto: for example.

A list of registry objects can be provided for data values if you wish to back up and restore should these change, or you can change the values in the registry so that permissions to change are blocked by the browser unique user name when shared by the same OS user name. WOuld you prefer a script to do this using a .reg file or some other method?

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I would like this to work on Windows7, and I am admin. Wouldn't associations for the http protocol be set for the "browser" rather than the "profile"? Like I said in my question, changing the "default browser" is not the same as changing the default instance of Firefox. –  BGM Aug 2 '12 at 20:13
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It seems that it is not possible without restarting all instances of Firefox. Here are a few Mozilla KB Articles that are relevant:

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