Hide titlebar for linux firefox in arc theme.

userChrome(linux_arc_theme).css

#titlebar .titlebar-buttonbox-container {
  display: block;
  position: fixed;
  z-index: 1000;
  top: -2px;
  margin: 0 1px 0 4px;
}
#titlebar .titlebar-buttonbox-container toolbarbutton {
  margin-left: -3px !important;
  margin-right: -3px !important;
}
#titlebar .titlebar-spacer, #titlebar .toolbar-items {
  visibility: collapse;
}
#titlebar {
  height: 0;
  min-height: 0;
}
#sidebar-header {
  display: none;
}
toolbarspring {
  min-width: 8px !important;
  -moz-box-flex: 8
}
toolbarspring:first-child {
  min-width: 82px !important;
  max-width: 186px !important;
  -moz-box-flex: 82
}