Firefox will start showing ads in their new tab page starting this week. You will see them as “Suggested Tiles.” There will be an option to make them go away. Mozilla wants to show that they can do “relevant advertising and content recommendations while still respecting users’ privacy and giving them control over their data.”
They have already been beta-testing this with some on the New Tab page. If you notice you have top sites you visit shown as tiles already. If they are not full, you may have already seen some suggested content filling in the rest.
Earlier this year, Firefox teamed with Yahoo! to make Yahoo! the default search engine. One of the upgrades at that time changed the default search engine. If you prefer another, you will have to go into your settings and manually change it.