Currently, signing in to Firefox with your Firefox Account  is synonymous with activating Firefox Sync -- all of the onboarding messaging is specific to Sync, and the very presence of an active signin session held by the browser is taken as a signal that it should sync its data to the associated account.
We're adding more Account-related features and services to the browser, such as a the Firefox Monitor protection report card (Bug 1559422). These features should be able to use the browser's sign-in state even if the user doesn't have (or doesn't want) Firefox Sync.
For more on the rationale, you can check out this motivational slide deck (but on the very explicit understanding that the proposed UX changes therein are for illustrative purposes only, and final UX is still in development).
This will be a significant refactor with lots of UI touchpoints potentially affected, so I'm filing a meta-bug to track all the moving parts. Broadly, we'll know that we've succeeded when:
- The "Join Firefox" flow from about:welcome allows users to set up their Firefox Account without enabling Sync
- The "Sign up to Monitor" button on about:protections can sign users in to Firefox and display their Monitor report without enabling Sync
- Users can easily enable Sync after the fact when they sign in via one of those flows.
 Note that "Signing in to Firefox" refers to credentials held by the Browser itself for its own use; it's entirely different from signing in to accounts.firefox.com on the Web, and we are not planning to conflate the two.