Firefox has the option to create multiple profiles and also has a profile manager, but its not directly available from the menu. Profile Manager and Selector is a Firefox Extension which can run profile manager inside Firefox and load additional profiles other than the current one in a new Firefox instance. With this add-on its easy to switch profiles inside Firefox. The add-on adds a profile manager button to the navigation bar.

Profile Manager Button

When you click this button, it loads the profile manager and we can load profiles in new Firefox window.

Profile Manager

By default, Firefox will automatically set the last profile you choose as the profile to load when it is restarted. You can set a default profile in the Options Dialog so that Firefox will always load that profile when started.

Settings Menu

Profile Manager and Selector is an experimental add-on, so you will need to login to download the add-on.

Download Profile Manager for Firefox