Doesn't sound like just grounds for termination (and since it comes in the wake of political pressure on the company to replace him, I don't take it that his resignation is "voluntary").
Nothing criminal or unethical about his political contribution. He's been "pinklisted".
The historical academics, political science majors, and media will never speak out against this "witchhunt" purging as they did of KNOWN Communists (sometimes working in support, knowingly or unknowingly, of the KGB and Soviet Union).
if he owns web patents he should block Mozilla from using them