>>614
-moz-border-image　に　fill を入れると解決する。
各所でfill抜けがあるけど、とりあえずスクロールバーについてだけ投下。

thumb[orient="vertical"]{
-moz-border-image: url("chrome://res/skin/aqua/scrollbar/thumb-v.png") 12 0 fill / 12px 0 repeat !important;
}
thumb[orient="vertical"]:-moz-window-inactive {
-moz-border-image: url("chrome://res/skin/aqua/scrollbar/thumb-v-inactive.png") 12 0 fill / 12px 0 repeat !important;
}
page:not([active="true"]) thumb[orient="vertical"]{
-moz-border-image: url("chrome://res/skin/aqua/scrollbar/thumb-v.png") 12 0 fill / 12px 0 repeat !important;
}
thumb[orient="horizontal"] {
-moz-border-image: url("chrome://res/skin/aqua/scrollbar/thumb-h.png") 0 12 fill / 0 12px repeat !important;
}
thumb[orient="horizontal"]:-moz-window-inactive {
-moz-border-image: url("chrome://res/skin/aqua/scrollbar/thumb-h-inactive.png") 0 12 fill / 0 12px repeat !important;
}
page:not([active="true"]) thumb[orient="horizontal"] {
-moz-border-image: url("chrome://res/skin/aqua/scrollbar/thumb-h.png") 0 12 fill / 0 12px repeat !important;
}