Maybe they could white label a password manager (Bitwarden?)
not sure starting at which version, but in android you can set bitwarden as a default password manager system wide which integrates pretty seamlessly with firefox and other apps. only place i know bitwarden doesn’t really integrate well with firefox is on Linux but that seems more due to bitwardens lack of interest to implement freedesktop apis.
i agree though with additional revenue streams, they need to break dependence on Google search engine revenue. maybe spend research on alternative monitization to ads and sponsored content that could be implemented at a browser level and split revenue between browser and websites.