No, it's not always positive. Mob mentality like this could be 'good' when applied to civil rights, the fact that he is not a minority and his views not popular in the zeitgeist makes him a good target. Of course this sort of things is considered bad when the target is a minority. Basically, what matters is whether or not you are part of a group that is perceived to have power. That will determine how the mob treats you.
I think that there's a real difference between types of behavior in this category. For example, if the same employees were calling him at night to yell the same message at him, I'm sure you'd agree that would be a terrible reaction. There are some pretty rude acts like that which are legal, but probably bad; e.g. the employees could go stand outside protesting loudly at his hypothetical kids' kindergarten, or spam all the Mozilla repositories with fake issues.
However, the specific behavior in the article does not seem like that. It is employees making tweets like "I love @mozilla but I'm disappointed this week" and "I'm an employee of @mozilla and cannot reconcile having @BrendanEich as CEO with our org's culture & mission. Brendan, please step down." That doesn't seem to be harassing or threatening or a witch hunt to me; it's individuals doing an honest job of saying their opinion.
Time will tell whether people start behaving worse than that, but I feel that so far this is an OK model of something you can do in a society when you feel strongly like someone has wronged you (but have no direct recourse.)
Witch hunts start with one person shouting that someone is a witch, and public opinion slowly grows and turns until someone is literally tied to a stake and has gas poured on them.
Nobody wants to literally burn someone alive. But at the same time, nobody wants to live with a witch in their community. Alone, nobody would go so far; but there's comfort and safety in numbers, and the more people support a cause, the easier it is to jump behind the masses and throw sharp objects without being singled out. If you'd like to see how an entire village can murder someone for, say, being a homosexual, google "necklacing".
Also, your idea of abusive behavior completely negates the real experience of people being harassed online. Women bloggers often get threatening letters and tweets that are hidden from view, telling them they should be raped, murdered, and worse. Even just a majority of people telling you you are a bad person is enough to cause severe depression and anxiety. Without experiencing it yourself, you really have no idea how horrible it can get. And if Brendan actually had a history of mental illness, this could easily drive him to kill himself, as many people have committed suicide from online bullying over the years.
Also: Are you saying these Mozilla employees were wronged? I'm certainly not aware if they were, so I don't see how any of this is justified.
'The mob' actually works on a variety of strange cognitive biases specific to group thinking. Part of it is who you are, part of it is who's leading the group and under what banner, if there's a perceived threat, and of course if there's some moral high ground that can be achieved. In general though, morality and critical thinking go out the window when large groups start agreeing with each other.