The next stable Firefox version, Firefox 67, will display a profile icon in the main toolbar to make Firefox Account features more accessible and discoverable in the web browser.
Firefox can be used without an account and that won't change; Firefox Accounts are needed to enable sync functionality in the browser and that is without doubt the main functionality right now. Mozilla started to use Firefox Accounts for other web services such as Firefox Monitor, Firefox Send, or Lockbox recently, and there is an option to use a "send tabs" feature to send open tabs to another device.
Mozilla ran a Shield Study recently to find out how it could improve the discoverability of the Firefox Account feature. The Shield study ran for 28 days; Firefox displayed an account icon in the main toolbar during the study and this resulted in an 8% increase in sign-ins to Firefox Accounts.
Users who participated in the study were asked to fill out a survey. 45% of users liked the profile icon (avatar), another 45% were indifferent to it, and 10% disliked it.
Mozilla decided to go ahead and implement a profile icon natively in Firefox to improve Firefox Account discoverability and use.