There’s an overview of several MLS related software projects at CloudServices/Location/Software - MozillaWiki. Typical users of MLS are Linux desktop applications and operating system services (like Gnome/KDE) and over a hundred different small IoT prototypes and explorations. There is also a client for Android Google Play services replacement microG (GitHub - microg/IchnaeaNlpBackend: Backend for UnifiedNlp that uses Mozilla Location Service for geolocation.).
For Firefox specifically, MLS is used by default for Firefox Aurora/Nightly users and Firefox users on all channels for some Linux distribution provided packages (Fedora, ArchLinux, etc.).
The situation with Firefox Desktop release and beta channels is complicated, as it touches business contracts and isn’t merely decided based on technical merit. As a single user, everyone can switch from the default geolocation provider to MLS by adjusting the “geo.wifi.uri” preference in about config and changing it to “https://location.services.mozilla.com/v1/geolocate?key=%MOZILLA_API_KEY%”.