It makes a fine browser extension. It ABSOLUTELY does not make good embedded code.

There is no good reason for it to be "core functionality" in a browser, other than the fact that Moz://a got a fistfull of dollars and foisted it upon us.

EDIT: For example, what would be a good use of core functionality, is if Moz://a included IPFS resolution and native support. That is a protocol level support, and thus would need native support rather than plugins. Plugins do indeed work, but require significant workarounds. For example, native could be ipfs://hash whereas the plugins need to specify http://127.0.0.1:8080/ipfs/hash