Firefox Customization - userChrome.css

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* to hide the native tabs */
#TabsToolbar {
    visibility: collapse;
}

/* to hide the sidebar header */
#sidebar-header {
    visibility: collapse;
}

* {
    font-family: "Hack";
}
window {
  font-size: 12px !important;
}

menubar, menubutton, menulist, menu, menuitem, textbox, toolbar, tab,
tree, tooltip
{
    font-size: 12px !important;
}

#sidebar {
  min-width: 100px !important;
  background: #bbc2cf; 
}