Works out really well, as Focus starts up extremely quickly (uses the native Android Webview), and I usually don't need each and every link in my browsing history, and it also means that my proper Firefox doesn't get filled up with random tabs that I didn't close out of properly.
Do you have a source for that? I see this and things like "Klar has the telemetry turned off by default" thrown around everywhere, but then other people again saying that there's no difference beside the name.
And while I assume, the F-Droid devs actual did do some research, I'm not aware of German privacy laws being so strict that proprietary dependencies or telemetry would have to be taken out. (Klar is the localized version for German-speaking countries.)
Also, I was able to use Firefox Focus on my phone and I don't have the Google Play Services installed, which doesn't say much, but that's what I assumed people meant with that.
* https://gitlab.com/fdroid/rfp/issues/235 * https://gitlab.com/fdroid/fdroiddata/merge_requests/2363
Basically, Focus uses GMS and Analytics. The telemetry is free software, but it is disabled by default for Klar (from what I can tell).