Firefox: Using multiple instances with different profiles?

You can do it from the command line:

firefox -no-remote -P profile_name

Try MultiFirefox:

MultiFirefox is a small launcher utility that allows you to run multiple versions of Firefox side-by-side. It helps you set up multiple profiles (one or more for each version of Firefox), remembers your last launch preferences for easy launching, has auto-updating built in for when we make improvements, and is 100% open source, MIT licensed.

Using an unx environement, you can use a command like :

env MOZ_NO_REMOTE=1 firefox -P Profile2

Using a windows or Mac OS X env, I don't know how you can do it