YJL

Believe me or not, I used to use Pentadactyl to change the background color of about:blank, which is not short:

js <<EOF
window.change_tab_bg = function(id, color) {
  var tab = tabs.getTab(id);

  if (!tab)
    return;

  tab.linkedBrowser.contentDocument.body.style.backgroundColor = color;
}
EOF

au PageLoad 'about:blank' js change_tab_bg(<tab>-1, "#333");
" PageLoad isn't fired when browser is launched
:if getBrowser().contentDocument.location.href == 'about:blank'
  js getBrowser().contentDocument.body.style.backgroundColor = "#333";
:endif

I dont know why I didnt think of something else, say userContent.css¹?

-moz-document url(about:blank){
  body {
    background-color: #333;
  }
}

A clean solution, and really simple. You can also use it for about:newtab, if thats your Firefox homepage, just replace the argument of url() with the URL.