The tl;dr here is simple: Mozilla made a hedge that Yahoo! would sell themselves to someone terrible during the period of their contract, Yahoo! did, and Mozill

The tl;dr here is simple: Mozilla made a hedge that Yahoo! would sell themselves to someone terrible during the period of their contract, Yahoo! did, and Mozilla collected on the proceeds to the tune of $350 million-ish USD. It was a gamble Yahoo! was desperate to take, so they did, and it didn't pay off in the long run (especially since for a large number of users it simply meant a one-time nuisance of switching the search provider back to a real search engine). But Mozilla made out like bandits on the deal due to their executives smartly calling the future of Yahoo!. Good on them.