Why would a search engine index something that is just a local easter egg in the client software? It's not a real protocol, it will definitely redirect to an http website.
It is, however, a valid observation for the use of the string within text. It likely means every search engine needs to now add another exception to their parser to treat moz://a as a word entity.
I think y'all are overthinking this. Mozilla is not changing their name to "Moz://a". It's a logo, that's it. No one has said "Moz://a" is going to be used in text.