For me, the best implementation of private alias is the Apple one: %randomwords%[at]icloud.com. It's way harder to wildcard block [at]icloud.com, as there are legit users of the icloud domain, than a wildcard block for: [at]mozmail.com.
Unfortunately, using the apple implementation is just one more stone into their walled garden. I really wish firefox could create a legit free [at]firefox (or something else) mail and then create this alias service as premium bundle. It would be way harder for services to start blocking it.
Furthermore, I'm not really excited to the overall direction that Mozilla is moving with its side projects:
1. They bought Pocket (which I loved) and now it's on life support.
2. They created an awesome private file sharing service (firefox send) and quickly butchered it.
3. They have a vpn that is simply mullvad with new clothes and fewer geographic availability. Why anyone would use it instead of mullvad is beyond me.
Mozilla needs some serious trust building before I trust it to manage several mail aliases for me.