I recommend Michael Widenius's video[1] about "Doing business with Open Source" it goes trough different models, and explains the reasoning behind MySQL license model, although I'm aware how different is a browser from a database it might help to think about this problem from different angles.
In the case of Firefox, if I'm not mistaken they do only consumer focus products, maybe they could release something that companies are willing to pay for.
They could captilize on the good reputation while it lasts, and maybe employees that resonate with Mozilla history will promote their products to their bosses for free.