How to change Firefox close-tab button size?

To change the size of the close-tab button in Firefox, you can use custom CSS. First, enable the "userChrome.css" file by navigating to about:config and setting toolkit.legacyUserProfileCustomizations.stylesheets to true. Then, create or edit the userChrome.css file in your Firefox profile folder and add CSS rules to adjust the button size, such as #tab-close-button { width: 20px; height: 20px; }. Restart Firefox to see the changes.