I would have to assume that any deal currently held by Yahoo would have to be upheld or compensated for if they sold to a third party. That's a contract and those just don't go away quietly.
As for one less search engine, true it might be an issue. But I'd rather they went with DuckDuckGo than Bing. Seems more in line with their stated philosophies.
I'm sure in the short term they would be fine, but when they need to renegotiate their contract eventually and then they'll be dealing with Google.
The organization that most of us know as Mozilla is heavily dependent on the revenue brought in by their search bar. They know this and are trying to diversify (ads in the browser, third party integrations like pocket and hello). I don't know how much DuckDuckGo could spend on a contract with Mozilla, but I doubt its near how much (previously) Google and Yahoo! have spent.
As for one less search engine, true it might be an issue. But I'd rather they went with DuckDuckGo than Bing. Seems more in line with their stated philosophies.