Hey man sienori, I am using your other addons for years and had a pleasant experience but recently discovered about this addon for Firefox and gecko-based Firefox browsers. Unfortunately, this works sometimes and has bugs. Sometimes it shows number, sometimes it doesn't regardless I am on a blank tab or any working tab. Also, allow us to increase the size of the number font and color. Keep it updated with new Firefox versions. Allow us to see more info when clicking on it like "Chrome Tab Counter" addon on chrome store does. Also, bring one for chrome users. Thanks.

No Preferences you can set.

Color options are ok (black text on blue/teal background) but could have more contrast.

Font size is kinda small IMO.

All in all, if all you want is a tab count then it will work but if you want configuration, better to keep looking.

クリックしたらタブのリストがでてくるとよりよいと思います。