The feature has always been in Firefox (I remember using it before Firefox 1.0), just well hidden. These days, I use the add-on profilist to make switching between profiles trivial in Firefox.
Profiles have always existed but I distinctly remember in the past getting "there's already an instance of Firefox running" when trying to have multiple open at the same time. proflist does make it easier to switch but there is no visible way to know which profile an open window is using.
