This article has been proven false on reddit (I can’t send links, go to firefox’s subreddit)
No keystrokes are being sent. In fact you can try going offline and if you type something, you might still get Firefox Suggest ads, because the links are stored locally and show up when you type them. It only sends some data when you actually click them, of course.
There is an online mode that can be enabled by the user tho, but its opt-in everywhere