Without knowing their reason, there are a few things tied to the org where multiple orgs make sense. If you do SSO for example you tie the org to a SSO provider, you can’t tie „just a few users“ to the SSO provider (afaik). The Firefox repo may have totally different authentication / users than the main Mozilla repo.
GitHub are terrible at this, because you can't have levels other than Org and Repository. And many things (SSO, visibility rules, common configs) are on the org level.
Unfortunately often the cleaner option is to create a separate org, which is a pain to use (e.g. you log in to each separately, even if they share the same SSO, PATs have to be authorised on each one separately, etc).
In Gitlab, you would have had one instance or org for Mozilla, and a namespace for Firefox, another one for other stuff, etc.
There is an “Enterprise” level above the org, but that obviously needs an Enterprise account. It lets you manage some policies across multiple orgs, including membership.
