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.