TLDR firefox is more than browser. And browsers are more than browsers were before.

inform - legacy links

Prevent Firefox from starting automatically

• HTTPS Everywhere
• uBlock Origin
• clearURLs

• dark mode for dark hipsters' eyes
  • DarkReader - universal “dark theme on every site” addon
  • reddit // how do you make new tab page also dark - various ways to make new tab page also dark
• tabs
  • ctrl+tab behaviour
  • always-right
• youtube
  • youtube-classic - speeds up stuff; is better
  • h264ify - better formats
  • sponsorblock - sponsorblock
• downloading
  • download helper

Firefox Account offers few interesting services:

https://send.firefox.com/

Firefox Send is a free-to-use, online file-sending service, able to handle files up to 2.5 GB

All 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/

https://support.mozilla.org/en-US/products/firefox/sync-and-save

https://support.mozilla.org/en-US/kb/send-tab-firefox-desktop-other-devices

Firefox Sync, originally branded Mozilla Weave,[2] 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.[3] 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.[4] It is also possible for the user to host their own Firefox Sync servers, or indeed, for any entity to do so.[5]²⁾

https://monitor.firefox.com/

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.[5][6] On desktop, Lockwise is simply part of Firefox;[7] whereas on iOS and Android, it is also available as a standalone app. ⁴⁾

https://support.mozilla.org/en-US/products/firefox/protect-your-privacy

https://support.mozilla.org/en-US/kb/manage-passwords-firefox-desktop-firefox-lockwise

• How to?
  • https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles
• Creating desktop shortcut to specific profile
  • http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• Plugin - Firefox Profile Maker (nice defaults) -
  • https://ffprofile.com

about:profiles