Streaming requires an active wireless connection for the entire duration of the movie. Downloading requires a wifi connection while the video downloads, but once the video is on the device, it may be watched later, when a wireless connection might not be available.
There's no word from Amazon about possible plans to allow Prime video downloads to older Fires or other devices.
- Prime videos may only be downloaded over wifi. If you have a Fire HDX with 4G, you cannot use the 4G connection to download Prime videos. You probably wouldn't want to anyway, as a single movie download would use a good chunk of your data allocation.
- Not all Prime videos may be downloaded. If a video is downloadable, there will be a Download button on the product detail page.
- You can download a Prime video on up to two separate devices at one time. If you have already downloaded a title to two devices, you need to delete it from one of them before downloading it to another device.
- You can have a maximum of 25 total Prime Instant Video titles downloaded at a time across all devices associated with your Amazon.com account.
- There is a limited viewing period for downloaded Prime videos, which varies by title. A typical viewing period is either 48 hours after you start watching the title or 15 or 30 days after the download, whichever is earlier. You'll need to check each video's product detail page for viewing period information, and plan your downloads accordingly.