Bug 1980297 - Avoid setting background of visually selected tab by default. r=dao,desktop-theme-reviewers a=RyanVM

This matches better the in-content styles.

Differential Revision: https://phabricator.services.mozilla.com/D259354

toolkit/themes/shared/tabbox.css

@@ -31,7 +31,7 @@ tab {
     background-color: color-mix(in srgb, currentColor 15%, transparent);
   }
 
-  &:where(:hover:active, [visuallyselected]) {
+  &:where(:hover:active) {
     background-color: color-mix(in srgb, currentColor 30%, transparent);
   }