143にアップデートしたら>>35さんと同じ状況になりました

怒られたので改行しています

/* FF88までのブックマークフォルダ 黄色(#FFCC00) */
.bookmark-item[container], treechildren::-moz-tree-image(title, container) {
list-style-image: url('data:image/svg+xml;base64,PHN2ZyB3aWR0aD0iMTYiIGhlaWdodD0iMTYiIHZpZXdCb3g9IjAgMCAxNiAxNiIgZml
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5PSIuMDUiLz4NCjwvc3ZnPg==') !important;
}