As GoOnThenDoTell pointed out, they could probably independently pay for Firefox developers but it'd probably be a messy approach.
Keep in mind that the Mozilla foundation gets something like $10 million in donations per year while the Corporation gets $500 million per year from Firefox search deals. So donations are a drop in the bucket compared to the existing revenue streams.
Keep in mind that the Mozilla foundation gets something like $10 million in donations per year while the Corporation gets $500 million per year from Firefox search deals. So donations are a drop in the bucket compared to the existing revenue streams.