A blank area on the left of the tab

[Xelliphine]

It's weird that there have a blank area on the left of the tab and it can't be closed or disabled, but it will disappear when the window is maximized.

This doesn't look consistent, so I hope it can be delete.

[Xelliphine]

Actually, I mean not delete this blank area completely, I wish it could be smaller than it is now.

[Agentvirtuel]

Hello

You can try
With the help of userChrome.css, you can find tutorials on the web

.titlebar-spacer[type="pre-tabs"]{display:none!important;}

Source, with the help of Browser Toolbox
https://firefox-source-docs.mozilla.org/devtools-user/browser_toolbox/index.html

[siffemcon]

Try

.titlebar-spacer[type="pre-tabs"]{width: 10px !important;}

in userChrome.css.

https://www.userchrome.org/how-create-userchrome-css.html

[Agentvirtuel]

Hello

To complement, if you wish to view, with the help of Style Editor
https://firefox-source-docs.mozilla.org/devtools-user/style_editor/index.html

Preview, userChrome.css
https://www.youtube.com/watch?v=4ujk-isbzF4

[Agentvirtuel]

Other illustration, userChrome.css

For information, a space for users css https://www.reddit.com/r/FirefoxCSS