Hello. Could you please provide more details about the issue ? If I understood correctly, just after installing Firefox for Windows, you are encouraged to connect (or register) to Firefox Sync. After your computer has applied to the Sync feature, you are proposed to register a second device. Again, that's a choice you have, not an obligation. So, what do you mean by "cannot use Firefox" ? Did you apply to the Sync process ? If yes, on your computer and your phone ?