Hi Frank, I haven't heard of that problem before. It sounds as though there may be a problem with the session history file.

Does it stay up-to-date with new pages you open throughout your session, and then Firefox opens all of those, twice? Or does Firefox seem to be rolling back to an older file or older duplicates?

There actually is a newer extension named Tab Session Manager. The legacy extension was just named Session Manager. If you are looking for a new session manager -- and that might be a good idea if we have to get rid of a corrupted session history file -- these are some to consider:

Use database storage:

• https://addons.mozilla.org/firefox/addon/tab-session-manager/
• https://addons.mozilla.org/firefox/addon/session-boss/

Use bookmark storage:

• https://addons.mozilla.org/firefox/addon/session-sync/
• https://addons.mozilla.org/firefox/addon/my-sessions/

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thanks so far for answers. I'll check that the next days and will come back...

So, here's the latest news: It seems, as if it only happens after an update of FF. Last week, no further problems occured but now - I had tot upgrade to 66.0.4 because all my AddOns did not work any more - it happened again: all windows and tabs were opened twice. It's really annoying...