Make sure that you do not start Firefox with the -no-remote switch or have set the environment variable MOZ_NO_REMOTE=1 (Control Panel > System > Advanced > Environment variables).

If you need to have more than one profile open at the same time then start subsequent instances with the -no-remote command line switch.
You can not send links from other programs to a Firefox instance that is started with the -no-remote switch. In such a case you need to drag links in a Firefox window or paste the link in the location bar.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

Very helpful. Thank you. I'll make my pinned taskbar Firefox open my main profile sans no-remote switch, and add a separate desktop shortcut with the -no-remote switch and -p to select other profiles.

However, and this is a nit-pick, since I'm so happy not to have to open new Firefox instances myself, but now the programs, like Media Monkey, opens its help in a new tab IN A NEW WINDOW. If it would open in the main (and only running instance in my test) window that would be even nicer.

Thank you.

You can check this pref on the about:config page (the default -1 equals 1).

• browser.link.open_newwindow.override.external

It has the same values as this pref:

• http://kb.mozillazine.org/browser.link.open_newwindow