Mozilla wants to keep Firefox profitable, and one of the means to do that is with ads. They can't just slap them all over the place, so they are going to be shown in the tiles, where they should be the least problematic.
Users don't like ads in their products, and they certainly don't enjoy them in an application that is built to surf the Internet, a place full of ads itself. On the other hand, Mozilla wants to make money, and it's not longer
making $240 million dollars a year from its Google partnership. Yes, you read that right, but that was all the way back in 2012.
Firefox is no longer ahead of Google Chrome, and Google is not giving Mozilla the same kind of cash that it used to, so they need to get that money through other means. Ads inside the browser seem like the only idea that's left on the table.
Users won't be thrilled
The community made it clear that they wouldn't tolerate this, but on the other hand, the community doesn't seem to have a lot of power. Suggested Tiles have been available in the US version of the browser for some time now, so this "feature" has been in the works for a while.
