If you're the owner of your company, the company won't fire you. You can still harm your company (and thereby yourself) by attacking your biggest customers on social media.
On his private twitter? While NOT posting about the company at all?
It was a customer of the company that got offended. Should he know every customer of the company? If the company has 1000s of customers, are they all off limits? Are you supposed to know who they are?
If you work for AT&T and an AT&T customer (of the millions it has) is a jerk or you have some political disagreement with their views, do you get to tweet about it on your personal twitter account? Should they be able to complain and get you fired?
I don't think anyone is debating VMWare's legal right to fire her, just the wisdom of such a decision.
If I was a big tech company especially one trying to find people interested in working lower down the stack, I wouldn't go around pissing off leftist furries.