Okay nevermind, literally minutes after posting this thread, I tried to "close other tabs" and closed the last tab, which closed the entire Firefox. Opening it again, it opened at my homepage as it should, and afterwards the problem was gone, no matter how many tabs I opened. So I suppose it was a bug with the particular session/tabs? I will write back if it happens again.

The last session will be restored automatically if the browser crashes or closes unexpectedly unless browser.sessionstore.resume_from_crash is set to false in about:config. It will also be restored if Firefox is left running during system shutdown unless toolkit.winRegisterApplicationRestart is set to false.

The last session will be restored automatically if the browser crashes or closes unexpectedly unless browser.sessionstore.resume_from_crash is set to false in about:config. It will also be restored if Firefox is left running during system shutdown unless toolkit.winRegisterApplicationRestart is set to false.

I got this bug again today after (probably) shutting down with my browser tabs open, then when I booted up and opened firefox, my tabs were restored. Closing and reopening at this point without manually closing all tabs continues this behavior. I tried disabling both of the settings you suggested, but it didn't help. I'll see if it happens again with these settings disabled, but there is clearly some bug causing this.

Restarting the browser after an update is meant to resume the last session. Make sure browser.sessionstore.resume_session_once is being set back to false, if it stays true for some reason, it will keep restoring the last session.