I worked there 11 1/2 years, and by the way was not fired. However, I saw many people fired. There were probably many more placed on a Performance Improvement Plan who subsequently quit rather than get fired.
My evidence is all the coworkers I saw who didn't get fired.
To clarify: I mean in the rank and file software engineering. If you make legal's job hard or give the company a public black eye, that can be a short path to dismissal.
(It's a big company, so I wouldn't be surprised if, perhaps, it also varies from department to department. I have no idea if the YouTube piece and the ads piece have the same policies).
You'd have to compare the firing rate at Google to other large companies, and that would be very difficult data to gather. Particularly when "firing" can take the form of the manager telling someone "you might want to start looking for another job."
I worked there 11 1/2 years, and by the way was not fired. However, I saw many people fired. There were probably many more placed on a Performance Improvement Plan who subsequently quit rather than get fired.