I don't think a single big donation is a good idea. We're so used to seeing extreme wealth we don't event question it.
Once a big donation is given, you get to wonder what sort of influence that person (willingly or not) has had on the project. A much better model is a large amount of small donations, the incentive becomes to serve the maximum amount of these people.
I too would prefer that the funding come from a relatively hands-off source, like some EU pot, if possible. But I think that nearly any (reasonably likely) funding source would be preferable to letting Servo development fail.
Once a big donation is given, you get to wonder what sort of influence that person (willingly or not) has had on the project. A much better model is a large amount of small donations, the incentive becomes to serve the maximum amount of these people.