How to create a fresh profile in about:profiles?

I was able to create a new profile using Firefox's alternative profile manager.

Important: Before you can start the Profile Manager, Firefox must be completely closed.

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1. Press Windows Key+R on the keyboard. A Run dialog will open.
2. In the Run dialog box, type in: firefox.exe -P
   Note: You can use -P, -p or -ProfileManager.
3. Click OK. The Firefox Profile Manager (Choose User Profile) window should open.

It might be easier to do it manually.

Go here: %APPDATA%\Mozilla\Firefox\Profiles

Then create a new folder.

Then edit %APPDATA%\Mozilla\Firefox\profiles.ini

And add a new section like:

[Profile1]
Name=my-test-profile
IsRelative=1
Path=Profiles/the-name-of-the-folder

You can also change this to 0 in the file to get it to prompt for the profile you want:

[General]
StartWithLastProfile=1