Add numbers to Firefox tabs for easier Ctrl+n tab switching

userChrome.css

/*  userChrome.css can change the Firefox UI.
    For details, see https://www.userchrome.org/

    Instructions
    ------------
    Save it to your Firefox 'Profile Folder'. To find that folder:
    enter about:support in the Firefox address bar and look for it in the list.
    Save this file to that folder. Then tell Firefox to load it:
    enter about:config in the address bar, and search for userprof
    and if that setting exists, set the following value to true:
    toolkit.legacyUserProfileCustomizations.stylesheets
    Then restart Firefox.
    */


/* Add numbers to tabs, to make switching
   with Ctrl/Cmd+[number key] easier.
   Thanks to https://github.com/tuomassalo/tab-numbering
   */

tabs {
    counter-reset: tab-counter;
}

tab:nth-child(1) .tab-label::before,
tab:nth-child(2) .tab-label::before,
tab:nth-child(3) .tab-label::before,
tab:nth-child(4) .tab-label::before,
tab:nth-child(5) .tab-label::before,
tab:nth-child(6) .tab-label::before,
tab:nth-child(7) .tab-label::before,
tab:nth-child(8) .tab-label::before {
    background-color: white;
    border-radius: 0.25em;
    border: 1px solid white;
    box-sizing: border-box;
    color: black;
    content: counter(tab-counter) "";
    counter-increment: tab-counter;
    display: block;
    float: left;
    font-size: 0.8em;
    font-weight: bold;
    height: 1.5em;
    line-height: 1;
    margin: 0 0.5em 0 0;
    padding: 0.1em 0.25em 0.25em 0.25em;
    position: relative;
    text-align: center;
    top: 0.35em;
    vertical-align: middle;
    width: 1.4em;
}

Instead of repeating the nth-child selector, this can be written in one line like this: tab:nth-child(-n+8) .tab-level::before {...}

I have tried to insert the modification you suggested but it doesn't work (when tested on Firefox).