The CBS All Access app, which was initially only compatible with the 2nd-gen Fire TV and newer Fire TV Stick with Voice Remote, has now added support for older Fire TV devices. The app still requires Fire OS 5, but since software update 5.0.5 has arrived on all Fire TV models, new and old, now all Fire TV owners can use the CBS app. Through CBS All Access, Fire TV owners can subscribe directly to CBS’s content through their $5.99 subscription plan which does not require a cable subscription.
As for the other three big US broadcast networks, the NBC app has always supported all Fire TV models. Even those still running Fire OS 3. The FOX NOW app is in the same boat as CBS, where it supports all Fire TV models but also requires Fire OS 5. Last and most certainly least, the WATCH ABC app still only supports the 1st-gen Fire TV.