Starting with the latest Firefox version 71 on openSUSE Tumbleweed Firefox is
restoring alle tabs from a previous login - despite the fact, that "restore
previous session" is switched off.
I tried to switch off all "sessionstore" options from about:config to no avail.

KDE is configured to start with an empty session.

I'm not sure, where to start debugging here, but in my profile a crash folder
exists and a file named store.json.mozlz4 within it. So I suppose Firefox isn't
shutdown correctly when logging off. But because
browser.sessionstore.resume_from_crash ist set to false, even in case of a
crash no resume should take place, right?

Stopping all Firefox processes manually after login and starting a single
Firefox process manually doesn't show this behaviour - only the configured
start page is shown in this case.