If you want to use Firefox Sync -- a service to synchronize settings between two or more Firefox installations -- you need to set up a minimum of two Firefox installations (typically on different devices). If you don't want to use Sync, don't use it. I'm not aware of any negative consequences of not using Sync. Work on MFA for Firefox Accounts is under way. I don't know what conclusions to draw from the following page about when it might launch, but earlier this year the goal was to release an initial TOTP implementation by next month sometime: https://waffle.io/mozilla/fxa?milestone=FxA-143:%20MFA