The problem is that the current system in practice legally protects certain opinions. E.g. If Eich was pro gay marriage and vocally so, firing him on that basis would have been very risky legally. Especially if he was gay, it would probably be impossible to fire him without losing an expensive lawsuit afterwards.
I don't think legally but more politically and PR wise. There a difference between firing someone for making donations to pro-X groups and firing someone for being X. In this hypothetical scenario, Eich would have to prove he was fired for being gay instead of for making a public donation that support a group and leads to potential blowback against the company. It's bad either way. Mozilla had a no win situation here. They got bad PR from both sides.
Forget about whether it has blowback against the company- It's totally legal AFAIK to fire someone for their "off the clock" speech for any or no reason at all.
In reality, though, I think a jury would be very skeptical of a company's claims that someone was fired for promoting the gay agenda (which would be legal) and not for being a gay person.
