This repository was an unofficial republication of the Firefox builds published by Mozilla in the form of an FDroid repository for easy installation and updating.
It was abandoned (with the latest version still lingering around until I've migrated all my own devices) because rfc2822 has a better repository for the same goal.
Mozilla server admins, please run something like that yourselves, it is not hard and well documented.
This issue is being tracked at mozilla support, and apart from this site there are also other workarounds that would IMO be best replaced with a Mozilla provided FDroid server.
--chrysn