Creating a new Firefox profile on Windows
From MozillaZine Knowledge Base
The Firefox Profile Manager allows you to create an additional profile while retaining your original one. This is useful not only for having separate settings for multiple users, but also for troubleshooting (see Note, below) . You should NOT delete any existing profile unless you are absolutely sure that you will never need it again. If in any doubt, then please leave alone.
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. All while still be able to go straight back to your original profile. 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.
The following assumes a standard Firefox installation on a standard Windows Operating System. If this does not apply to you, please see Profile Manager
The 'Firefox - Choose User Profile' window will now appear whenever you start Firefox. 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 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.
See also - Uninstalling add-ons #Global extension