Usually you only have one browser profile that you use for everything - you have your passwords on it, save your tabs and bookmarks there and also pile up history and cookie data. If for some reason you want to divide some of this stuff between user profiles and use those with different command line arguments, Firefox let's you do exactly that.
Before you proceed, take a look at the warnings on the Mozilla page - I had no issues with it however.
To access this feature, close all Firefox windows. Then open a cmd or terminal window, depending on your operating system, and open Firefox with the -P argument (if the Firefox location is not in your PATH variable, you have to direct the command line window to the application's directory first - you can do that using the cd (change directory) command, for example like this:
cd "C:\Program Files\Mozilla Firefox\"
The Firefox Profile Manager will open:
Create a new profile using this manager. Afterwards, whenever you run Firefox without any argument, the highlighted profile will be loaded. You can determine which profile should be used by using the -p argument followed by the profile name, following will load the profile faqforge for example:
firefox -p faqforge
To ease profile selection, you can create multiple Firefox icons or symlink and assign them the appropriate command line arguments to open with.