It's actually documented that Firefox does have outside contributors, even people writing code: https://blog.mozilla.org/community/2019/04/30/firefox-67-new...
So yes, that happens, and if that's not as easy as we could have hoped, probably help is welcome with that as well.
Big and complex projects generally have higher standard of effort necessary to get the leg in. The comparison to Chrome was that it's a project comparable in complexity, and yet Firefox is easier and faster to build.
I'll take Mozilla over Google every day, but if Mozilla presents itself as open to community, it also lends itself to criticism that Google avoids altogether.