Hellllooo Mozilla Connect! We want to thank those folks (2,852 of you!) who participated in the Firefox User Research survey on feature priorities we posted here last month. If you recall, this survey showed participants multiple sets of 3 random browser features, then had participants pick whether...
You copy/make a shortcut to Firefox. Then edit the target to: "C:\Program Files\Mozilla Firefox\firefox.exe" -P ProfileName -no-remote where ProfileName is the name you set during the (convoluted) setup.
-no-remote isn’t needed on the first profile you launch but is needed on subsequent profiles you want to run concurrent to the first one.
Edit: I have up to six profiles I run at once, and I also use the Container Tabs extension.