I think MrEldritch is referring not to Firefox's built-in password manager, but third-party password managers such as Bitwarden, KeePass, and LastPass. Those rely on add-ons for browser integration.
One you have targets mapped correctly keepass2android is great and I now that I've gotten used to it, I prefer the system ui password filling for everything including the browser. Also installing the keyboard extension is great and makes for an easy way to quickly access logins.