Note: If you are creating a new profile for troubleshooting purposes then do not install any extensions or themes to it. The last selected profile will then start automatically when you next start Firefox and you will need to start the Profile Manager again to switch profiles. If you decide that you no longer want to see this window, check the 'Don't ask at startup' box the next time it appears. The 'Firefox - Choose User Profile' window will now appear whenever you start Firefox.
On Windows 7 or Vista, press "windows key+R" if you haven't enabled the "Run command" option for the Start menu (details here)
It is suggested that you back up or export bookmarks from your old profile now, as a precaution, before moving on to the following stages. All while still be able to go straight back to your original profile. You can then use your newly created additional profile just as it is, import bookmarks to it immediately or transfer some/all of your original settings over to it. If in any doubt, then please leave alone. You should NOT delete any existing profile unless you are absolutely sure that you will never need it again. This is useful not only for having separate settings for multiple users, but also for troubleshooting (see Note, below). The Firefox Profile Manager allows you to create an additional profile while retaining your original one.