This [[https://support.mozilla.org/en-US/questions/1195017 previous question]] previous question is archived, but it exactly describes and explains the problem: when only pinned tabs are open when Firefox closes, then on startup - when restoring the previous session is enabled - the previous session is restored ''and'' a new tab is created. This is annoying, so I "fixed" it: https://gist.github.com/adeilt/6ed372f4d8a7c54f9df49a3ca1adc4b2 That's a gist of an html file that will automatically close the tab it's in. It does involve using about:config, so use appropriate caution. Just set that to be your "homepage" in the Firefox settings and the new tab will likely disappear before you even notice it. You can tweak the delay if it's not fast enough or you want to savor the impending doom of the offending tab. :)