They aren't openly removing top executives because of their political beliefs.
I removed FF from all my devices.
No, they just would never hire them in the first place.
You might find it enlightening to go here. Put in some browser-related employer names and compare Support vs Oppose.
Or you could just download the whole list via the Download CSV link. But, before you load it up in Excel, be advised 9 Microsoft employees supported Prop 8, whereas 178 opposed the measure.