The parallel there would be giving away @mozilla.org addresses. @firefox.com to me feels somewhere closer to @gmail.com, both being products, not companies.
Only that Firefox is a much more valuable brand than Mozilla
In countries where GMail is/was a pre-existing trademark Google is/was giving away @googlemail.com, didn't seem to devalue the brand.
I’m not against that. They could sell @firefoxmail.com addresses