Linux Mint has put Google custom search into the distribution in order to raise revenue. It's in firefox both from the address bar, the search box, and in the quick search from the 'start' menu. It's very annoying, and it isn't just a matter of opting out. Mint doesn't support an easy way to remove it from all three locations without getting technical.

I used mint for a couple of months, the custom search engine came back every time I updated a browser, and it was very annoying to reset settings. It was a deal breaker for me.