firefox-esr is not apackaged on Alpine for aarch64 right now. As firefox is just a demo in the current state, it will be left to the user to add it to his installation with the --add flag, so this will not break the install step anymore.
The firefox package is currently broken in armhf Alpine, and it looks like fixing will take a while (depends on packaging rust for armhf on Alpine): https://bugs.alpinelinux.org/issues/7409 https://github.com/alpinelinux/aports/pull/1578#discussion_r119988654
Later, the aports folder will probably get split up in its own repository. But right now this is simply convenient.