Mozilla cannot enable one provider by default. People already complained that Cloudflare was initially the only choice.

Users at the moment are expected to choose their provider anyway.

This deal is about Mozilla picking Comcast by default for Comcast customers. This is essentially as if they'd be using the network's default, because Comcast is the network's default already, being what people get via DHCP.

They can always choose a different provider. And Mozilla apparently struck a privacy deal with them too.