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Not great, but I'm happy to promote Firefox and have done so for a long time before the referrals were available. So if I make a few extra dollars a month out of it, I'm quite happy.
The CTR does seem a bit low, especially as with modest traffic you wouldn't expect to refer that many people.
I suppose it's money for nothing for those who already promote FF as an alternative to IE. It does include the Google toolbar though, which I'm not a real fan of.