I also run a B2B SaaS and I have found that over time the customers who I think are going to be terrible for paying, typically tend to be very good at paying and the customers who I think are going to be very good for paying (in that they have good standing in the community in things like that) tend to be pretty bad at paying on time (and with NET30 and all)
Ultimately I don't really understand why I shouldn't be paid on time (I've got to live too, and I have suppliers likewise) so I think this is pretty fair game, I sold you a product under the promise that you would pay in 30 days, if you're incapable of doing that (ignoring exceptional reasons) I don't necessarily want to go and lend you extra credit just because you're Mozilla
Having a terrible accounts receivable department isn't a exceptional reason.