When microb is default wish to have running the browser non-visible in the background it all the time If an other browser than microb is my default browser microb should not run as background process. In this case it's okay for me to wait a little bit longer after starting microb explicitly
Everything seems to work as expected.
I was surprised because Opera was already my default browser. In step 1. I used apt-get purge to unistal browser-switchboard and expected that all config fles are deleted too.