I totally liked the idea, since I am a fan of the concept of configuring the browser after the assignment at hand, until I checked the implementation of it in the folder-structures.
Some user (please read I), have several user profiles in a main folder not in C:, so when applying this new concept, opening one user profile and creating a new profile within the user profile, FF copies the content from the main folder to the appdata-folder and creates a mess.
It would be better if the original user-profile stayed in its original-folder and future profiles could be assigned to subfolders of the original user-profile or folder by choice, hence making it possible to migrate several old user-profiles to this new great idea of having several profiles in one user-profile with several profiles in it.
Best Wishes,
/Alex25