Mozilla could keep building FF without Google, but it would require that 90% of the C suite get laid off and dedicated focus from the company...
If they lost the Google revenue they wouldn't just have to fire "the C suite" but lay off most of their engineers. An independent browser engine is a project with code in the tens of millions of lines, you obviously need to pay hundreds or of engineers to work on this, which is why there's pretty much only three competitive ones, all maintained with significant resources.
It's a large code base, but it doesn't require hundreds of developers working concurrently to keep it up to date. Web standards move fast, but not that fast.
Take a look at Ladybird. There's a browser being built from scratch with a small team (less than 10?)
If Mozilla fired 90% of it's employees and kept the 10% to actually work on Firefox, it could be a great browser.
Ladybird is so, so far away from being comparable to Firefox. It only supports a fraction of modern websites.
[1]: https://bugzilla.mozilla.org/describecomponents.cgi?product=...
In 2009 they had a 30% market share.
Now they have a 5-6% market share.
4/5ths of their market share, gone.