There is no freedom of speech when it comes to the corporate envirronment, and even less when you are a public figure of a business.
So this is not a freedom of speech issue. Eich is free to say whatever he wants, and Mozilla is free to decide wether what Eich says is compatible with its CEO position or not.
The freedom to assemble doesn't mean that private owners can't keep you from assembling on their property. The freedom of press doesn't mean that the editors can't censor your work. The freedom to bear arms doesn't mean that private businesses can't bar you from the premises.
There are only 21 states in which this is the case.
Just about every aspect of our lives are ruled by corporations, and definitely every avenue of communication. I absolutely refuse to accept that corporations can control what is acceptable for me to say. This is exactly the road you guys are going down when you make statements like this.
How was that taken away from him? The freedom of speech doesn't include the ability to hold onto a specific job when you exercise your speech rights. That idea is downright silly.