Not to sound like snark, but where does the money even come from? I feel like you've ruled out either donors or investors, which really makes me confused over what's left.
A significant part is default search engines sharing revenue - primarily Google at the moment. There are also regular customers, for products like Mozilla VPN or Firefox Relay. And there are donors for the non-profit entity as well, as well as for the entity managing Thunderbird.
But the big money maker is Google's revenue share.