If you like to use several different profiles of Firefox then you'll know that switching between them isn't as easy as it should be. Profile Switcher makes it very easy.
Profile Switcher adds the profile name to the window title and adds options to launch other profiles. It also works works with Thunderbird as well. Profile Switcher adds new profile names to Firefox so that when you launch Firefox, you will be asked which profile you want to launch. Alternatively, you can switch by using the File menu in Firefox.
The extension has some preferences which allow you to choose what to do when you launch another profile i.e. close the one in use, don't close it, ask every time. If you choose to run the profile manager in safe-mode, your current profile will be always closed. It also adds the name of the profile in use in the titlebar (on Thunderbird but not in Firefox) and also as a tooltip in the statusbar.
Profile Switcher is a very useful Firefox and Thunderbird add-on for anyone that uses multiple profiles in either.