TLDR firefox is more than browser. And browsers are more than browsers were before.
Firefox Account offers few interesting services:
Firefox Send is a free-to-use, online file-sending service, able to handle files up to 2.5 GBAll files are encrypted before being uploaded and decrypted on the client after downloading. The encryption key is never sent to the server.1)
There is also cool sidebar plugin - https://addons.mozilla.org/en-US/firefox/addon/send-to-fx/
Firefox Sync, originally branded Mozilla Weave, is a browser synchronization feature for Firefox web browsers. It allows users to partially synchronize bookmarks, browsing history, preferences, passwords, filled forms, add-ons, and the last 25 opened tabs across multiple computers. The feature is included in the desktop Firefox browser, Firefox for iOS, and Firefox for Android.It keeps user data on Mozilla servers, but according to Mozilla the data is encrypted in such a way that no third party, not even Mozilla, can access user information. It is also possible for the user to host their own Firefox Sync servers, or indeed, for any entity to do so.2)
It informs users if their email address and passwords used has been leaked in data breaches, using the database provided by Have I been Pwned? (HIBP)3)
Firefox Lockwise is the password manager of the Firefox web browser. On desktop, Lockwise is simply part of Firefox; whereas on iOS and Android, it is also available as a standalone app. 4)