Out of most important domains and services run by Mozilla, only a few have IPv6 support active with announced AAAA DNS entries.
These that do support IPv6 are:
But these are legacy IPv4-only:
As you probably noticed, this list is considerably longer, contains some of the most important domains and these are just the ones I were able to find by following a few links on the main mozilla.org home page. There's probably a lot more, including underlying API domains that browsers and services make calls to.
Please, for the sake of better internet, enable IPv6 support at least on most important, highest traffic domains and APIs your services use.