Firefox focus also has the epic feature of not being able to add new bookmarks for some goddamn reason and also needing to close the app if the webpage doesn’t let you scroll up.
Firefox Focus is an Android browser that's always in Private Browsing mode, and has tracker blocking and content blocking built-in. It does not have all of the advanced features of the regular Firefox app.
Firefox Nightly is the cutting-edge development version of Firefox. This is built from the latest codebase automatically on a nightly base, as the name implies. If you want the newest features ASAP and don't care about bugs or stability issues, this is the one for you. Personally I would recommend using Firefox Beta instead, which sits in the middle -- more tested than Nightly, more cutting-edge than regular Firefox.
Firefox Klar is the German-language version of Firefox Focus. I think the only technical difference is that it has telemetry disabled by default. (You can turn that off in Firefox Focus as well, under Settings > Privacy & Security).
Never heard of Iceraven before. Apparently it's a fork of Mozilla's development version of a new Android app codenamed Fenix. Hopefully someone else can chime in with more insight into these two.
If Firefox focus Android has user-friendly tab management, it'll be my main browser. Its native ad blocker is 👍. 1 may say that there's Firefox Android + Ublock origin. But I <3 Firefox and I'm :) they have a native ad blocker.
Given Germany has some of the strictest privacy laws, I'm betting Klar is the honest version of focus. The fact that both exist separately makes me question Firefox's commitment to privacy.
Klar by Firefox is the German-language version and only available in Germany, Austria, and Switzerland. It includes the same features as the English-language app Focus by Firefox.
Why did we change the name/replace the app?
The publishing and media company Burda, which the news magazine FOCUS belongs to in Germany, is one of our partners. We therefore decided to replace our app "Focus by Firefox" in Germany, Austria, and Switzerland by "Klar by Firefox".
If you think it is suspicious you should be able to spot it in the open source repository. Otherwise see the other, and probably only, reason for the different name.