>>105587100
I just picked the css manually and could find something (picrel is the result).
Try this:
/* Make the background tabs more visible: */
.tabbrowser-tab:not([selected="true"]) .tab-background {
background-color: rgba(207, 207, 216, 0.2);
/* border: solid 1px rgba(207, 207, 216, 0.2); optionally customize the border */
}


If you want to also change the border-color for inactive tabs, just uncomment the border line.
I'm assuming you already have the userChrome.css in the correct folder and toolkit.legacyUserProfileCustomizations.stylesheets enabled.