I think you are comparing apples to oranges here. Linux is made of many distros, each one with their own strong points and features. Many different maintainers matain them. There is no single point of funding for them.

Mozilla on the other hand makes basically one semi well-known product (and other even less known stuff) and gives it away for free.

If tomorrow Google pulls the plug, who will pay for the salaries of the engineers who maintain Firefox? The general public does not care if Firefox lives or dies. In my circle of friends and family, I am the only one who uses Firefox. Most people are on Chrome or Brave. That's it.

Someone in the comments above mentioned that Mozilla could release a paid version for Enterprise customers, imitating Red Hat in a way, but I am highly skeptical that Enterprise customers in times such as these will be willing to pay for something that they can get for free from Google or Microsoft.

I guess we will have to wait and see.