Firefox currently selected tab at the top will show there is a line, I feel ugly, as shown:
In the address bar about:support, and then click to open the directory .
In this directory create a chrome folder (if not), then enter the chrome folder, create a file named userChrome.css file, open the file type:
#TabsToolbar .tabbrowser-tab[selected] .tab-line {
display: none !important;
}After saving, restart Firefox.
Renderings:
