ro_em wrote:

can I bring it back, preferably with the classic theme restorer? 

I don't see that option in Classic Theme Restorer 1.1.8. You can request the feature.

• http://forums.mozillazine.org/viewtopic.php?f=48&t=2827985

1. Install Stylish and restart Firefox when prompted.
   • https://addons.mozilla.org/firefox/addon/stylish/
2. Click the ≡ Menu Button and choose Add-ons.
3. In the Add-ons Manager, click User Styles on the left.
4. Click the Write New Style button at the top. Paste the following in the text box, give the style a name, then click the Save button.

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

.tabbrowser-tab .tab-throbber:not([busy="true"]) + .tab-icon-image:not([src]) {
  display: -moz-box !important;
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAtUlEQVR42t2TQQrCMBBFe4HqungBV8USjC01iKt2UVBcuPAwHkAKRSwFN141zi/DULpKiCsXL8kw/3+SkETW2iDGQZeHdabL+2ZbfKiOMYPL9bYDXEsPWngkQOX7Zz+8T7Q2EGFmVoyZ9qCFRwI4PYHAkQQeCZB0P8xsB34B8AQF/PYIdXOufAPg+aM7CN9Bbo4vevPa1QwtPBLwaLvxbbt+JmjhkQBiQaSE4loxS0bNeil7oi+lcro7fpDjUgAAAABJRU5ErkJggg==") !important;
}

If you don't want to install an add-on, you can use the userChrome.css file instead, but I don't recommend it.

• http://kb.mozillazine.org/UserChrome.css

This worked and brought back the icon. Thank you so much!

If you would prefer another icon then you can use this code (works for more cases):

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

.tabbrowser-tab .tab-throbber:not([busy="true"]) + .tab-icon-image:not([src]) {
  display: -moz-box !important;
}

#tabbrowser-tabs .tab-icon-image:not(src),
.sidebar-placesTreechildren::-moz-tree-image(leaf),
.bookmark-item:not(src):not([container]):not([type]){

 list-style-image: url("chrome://global/skin/icons/folder-item.png") !important;
 -moz-image-region: rect(0px,16px,16px,0px) !important;
}