I can't see how this partnership is not acceptable. Firefox is an NGO for the open web not one for human rights.
On a personal level, many people chose their friends/acquaintances based on these sorts of things. On a consumer level, there are many examples of popular boycotts (for example see http://en.wikipedia.org/wiki/Nestl%C3%A9_boycott or http://en.wikipedia.org/wiki/Anti-Apartheid_Movement ). On a company level, many companies consider things such as environmental or social duties when deciding who to do business with.
In all cases, it could be because you hope that such a decision will cause change (e.g. organised/promoted boycotts) or it could be simply that you don't want to support something bad, even if knowing that refusing to spend $1 on some Nestlé chocolate (for example) probably won't change a thing.
Nobody fight for everything, choose a niche and fight for that.
My point is that just because human rights is not their goal, it doesn't mean it can't be a consideration. And if Foxconn, rather than being a company with a somewhat poor track record regarding its staff, was a company known for funding terrorism, arms sales, etc. then I don't think anyone would be arguing that people shouldn't do business with them.