>>791
userChrome.css で appearance: none
background-color 以外も解除されるので自分で適当に設定する

@-moz-document url(chrome://browser/content/places/bookmarksSidebar.xhtml),
url(chrome://browser/content/places/historySidebar.xhtml)
{
#search-box {
appearance: none !important;
background-color: green !important;
color: yellow !important;
border: solid thin var(--input-border-color) !important;
border-radius: 2px !important;
padding: 2px 3px !important;
}
}