Now audio seems to work simultaniously in firefox and strawberry.

But pulse is active. Is that a problem for sound quality despite the fact that I have chosen "Output.... ALSA" in Strawberry?

pipewire

Not sure, maybe look here:
https://wiki.debian.org/PipeWire

Do you know if it's the same syntax for pipewire?

#!/bin/bash
systemctl --user mask pipewire.socket
systemctl --user stop pipewire.service

So pulse audio is blocking palyback. My solution:

I created two Desktop-Shortcuts with two scritps behind. Script one pulse-stop.sh:

#!/bin/bash
systemctl --user mask pulseaudio.socket
systemctl --user stop pulseaudio.service

And pulse-start.sh:

#!/bin/bash
systemctl --user unmask pulseaudio.socket
systemctl --user start pulseaudio.service

So when I want to play audio in Strawberry, I click the stop-pulse-button and when I'm finished, I press the start-pulse-button. Not perfect but works