The latest version of Firefox Klar/Focus seems to have some stability issues, see for example [here]. A workaround for that one might be to use the Android app switcher instead of the buttons on the persistent notification of Firefox Klar to switch back to the app. Let us know if that works for you!
We are using the Firefox Klar version from F-Droid. They have recently updated from version 6 (from last year) to the latest version 8.0.15, which comes with a lot of changes internally. Despite the bug you are experiencing, it is very important to us to update to the latest version of Firefox Klar, to stay up-to-date in terms of security of the browser that we preinstall for you.
As soon as F-Droid provides a new version of Firefox Klar, you will be able to install the update on top of the version we preinstall (because our preinstalled version also comes from F-Droid). We will of course also include it in our next updates. I hope that upcoming versions will fix the issues you are experiencing!