I'm pretty uncomfortable with the idea of a boycott for this, provided I understand everything correctly and the problem here is just that Mozilla promoted a guy who has hateful views and makes political donations but does not leverage his position with Mozilla to influence politics in any interesting way (he might still use his salary, but I think that's rather different).
A commitment to substantive free speech requires that we not go after people for political disagreement and that we treat political engagement the same regardless of the views of the people doing the engaging. That is, it's fine to want to go after the Koch brothers because they're trying to control the entire political system and nobody regardless of their political views should be doing that, but even though this guy is going to have had a bigger impact on politics than the average person just because of how much money he can bring to bear, the kind of thing he's done isn't (procedurally) irregular and isn't the sort of thing most people usually get upset about. This is clearly about his political views rather than his method of advocacy.
And maybe there are exceptions around the edges. Maybe the right response to neo-Nazis is to shun them out of society. But let's have some perspective here - there's a pretty big evil-ness gap between opposing gay marriage and wanting to commit genocide. The odiousness of the view needs to overcome the general importance of not punishing people for their political views and activity. But what distinguishes gay marriage as an issue is not so much that it is vitally important to the welfare of gay people but rather that it is absurdly easy to land on the right side of - there's just nothing resembling a reasonable argument against it, such that it's very easy to label opponents bigots. It's a highly salient issue because the answer seems so obvious (to be fair, it seems obvious to people on both sides) rather than because getting it right in the short-term is very important. If you just wanted to punish people who support policy that makes innocent people worse off, because it does that, you should probably start with supporters of the Republicans' economic agenda rather than their social one.
To be clear, I'm not making a "you have to care about these eight other things which are more important than this in order to be allowed to care about this" argument. I'm pointing out that the sorts of principles being applied to justify a Mozilla boycott here (or similar) are completely opposed to existing norms of not punishing people for political disagreement. The arguments would justify boycotts for just about any significant political disagreement, if someone were motivated to do that. And that's a problem because we like these norms and it's very important that we not have boycotts for just about every significant political disagreement. It's a categorical imperative thing. We do need to tolerate a certain amount of intolerance, if it's expressed in a procedurally acceptable way.
