Amazon has teamed up with IMDB (insofar as Amazon, which owns IMDB, probably said "Hey, IMDB! We're going to do a thing) to bring free movies to the Internet Movie Database.
It's a free service called "IMDB Freedive." (Which first was rumored to be coming back in August.) And on it you'll get movies like Memento and True Romance, TV shows like Fringe and *Quantum Leap (oh, boy ...) and IMDB originals like The IMDB Show, IMDB Brief and Casting Calls. There's actually quite a bit of content on there, and the Amazon Prime Video app already is pointing folks toward the ad-supported service. It's a U.S.-only thing for now. If you still want to buy things, you can. This is just a free option.
Amazon says that Freedive works on Fire TV devices — and indeed it does — but it also works just fine through the Amazon Prime Video app on all the places we've tried it — Android, iOS, Roku, etc.
In addition, Freedive works with Amazon Alexa. So you can tell your Echo device — or just use the Amazon Voice Remote — to tell your Fire TV to "Go to Freedive" and it'll take you straight there.