Für alle Fälle, teste mal:
CSS
@-moz-document url("about:addons"){
#category-dictionary{
list-style-image: url("data:image/png;base64,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") !important;}
#category-dictionary .category-icon {margin-left: 6px !important;}
}
Mfg.
Endor