I hate to say it, but the firing is justified. If your job requires you to do something, and you refuse to do it, that justifies the firing.
That’s not to say that Google is in the right, either in providing the service to Israel or in the firings. Justified doesn’t mean it’s right, just that it’s a valid reason.
And if you’re working for a company that goes so far against your principles that you’d protest against them, even if they don’t fire you, you should certainly be looking for work elsewhere. Especially if the protest doesn’t work.