I've asked about this here before (how to support MoCo and Firefox with money), but there doesn't seem to be a way to do that so far. This is one of the reasons Mozilla Corporation has been looking at other services, like the VPN service we've heard of, to get more revenue. Mozilla Corporation also laid off several people from the Firefox team recently because...not enough money, in very simple terms! This is not to say that one must not donate to Mozilla Foundation. Just be aware of where your money can go and what it can be used for.
On the other hand, donations to Mozilla Thunderbird go directly to Thunderbird development, maintenance, etc.
Edit: Removed an erroneous statement about the search engine partnerships.