(guide) Firefox 60 dark newtab.md

Open about:config.

Set toolkit.legacyUserProfileCustomizations.stylesheets to true.

Open about:profiles.

Click "Open Folder" on the "Root Directory" row on the profile in use.

Create the folder "chrome".

Create userContent.css, and add:

@-moz-document url(chrome://browser/content/browser.xul)
{
    browser[type="content-primary"],
    tabbrowser tabpanels,
    #appcontent > #content {
        background: #111 !important;
    }
}

@-moz-document url(about:blank), url(about:home), url(about:newtab) {
    html, body {
        background: #111 !important;
    }
}

Create userChrome.css, and add:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
/*.tabbrowser-tabbox { background-color: #111 !important; }*/

#browser vbox#appcontent tabbrowser,
#content,
#tabbrowser-tabpanels,
browser[type="content"][primary="true"],
browser[type=content] > html {
    background: #111 !important;
}

Customize to your desire.

Inside about:profiles, click "Restart normally".

Restart after every customization.