Desktop:

• damn nice
• good Flatpak version playing everything
• all extensions work without weird restrictions
• only Forks that make sense, Librewolf and Tor Browser
• loads of search engines
• arkenfox user.js
• UserChrome.css, in theory, if it was easy to create one. Only SimpleMenuWizard currently to make it less rediculous to use
• also Thunderbird uses Firefox ESR, and its the only good Mail program I know

Mobile

• mull, prehardened
• support for a looot of addons using a custom Addon collection
• offline translations using FF Translations, literally the only app I know that does this and its so damn critical
• dark mode without tracking
• ublock, noscript
• lots of search engines possible
• privacy

I dont use Firefox as it comes on any device, as this is unusable. But with hardening its simply the most secure browser