beaconDB is a drop-in replacement for MLS, which uses the same format request that’s used by Mozilla’s Ichnaea.
The source code is available on Codeberg: https://codeberg.org/beacondb/beacondb
You can contribute to the project by using an app like NeoStumbler (GitHub) or Tower Collector (GitHub) to submit location reports. NeoStumbler does Wi-Fi, Bluetooth and GSM, while Tower Collector can only do GSM cell towers. Both are FOSS and available on F-Droid.
It is also recommended by the GrapheneOS project: https://grapheneos.social/@GrapheneOS/112759509558471713
https://grapheneos.org/articles/positon-location-service
Just keep in mind that it’s still in relatively early development, which is why it really needs contributions.
Why is this needed? There’s a reason for Mozilla cancelling their service.
So, you are willing to give your location to Google anytime any app asks for the location permission? A lot of people is not. And there’s been efforts over time to get privacy-friendly location services. So there’s clearly demand for this, even if it’s still niche due to lack of knowledge.