Maybe contact the developer of the "Undo Closed Tabs Button" extension and ask them to look into this.

The icon might be transparent and the expected white background will make the icon look quite different when the background is black. I don't if you can possibly use code in userChrome.css to set a lighter (white) background for this button.

• https://www.reddit.com/r/FirefoxCSS/
• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html

I chose the easier solution: downloaded another similar extension with a visible icon and I'm going to do the same with other extensions Thanx for your support anyway Regards Georges