フラットなタブ、ツールバー、メニュー

にしてみた。

フラットタブ:

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

.tabbrowser-tabs {
	background-image: none !important;
}

.tabs-bottom {
}

.tabbrowser-tab,
.tabbrowser-tab > *,
.tabs-left {
	-moz-appearance: none !important;
	background-image: none !important;

	-moz-border-top-colors: none !important;
	-moz-border-right-colors: none !important;
	-moz-border-bottom-colors: none !important;
	-moz-border-left-colors: none !important;

	border: none !important;
}

.tabbrowser-tab,
.tabbrowser-tab > *,
.tabs-alltabs-stack {
	max-height: 24px !important;
}

.tabs-left,
.tabbrowser-tab,
.tabs-alltabs-stack {
	margin-bottom: 3px !important;
}

.tabbrowser-tab > * {
	margin-bottom: 0 !important;
	font-weight: normal !important;
}

.tabs-left,
.tabbrowser-tab > .tab-image-right {
	border-right: 1px solid threedshadow !important;
	margin-top: 1px !important;
}

.tabbrowser-tab[selected="true"],
.tabbrowser-tab[selected="true"] > * {
	background-color: threedshadow !important;
	color: threedhighlight !important;
}

フラットツールバー:

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

toolbox,
toolbar,
toolbarseparator,
statusbar,
statusbarpanel {
	background-image: none !important;

	-moz-border-top-colors: none !important;
	-moz-border-right-colors: none !important;
	-moz-border-bottom-colors: none !important;
	-moz-border-left-colors: none !important;

	border: none !important;
}

toolbox {
}

toolbar {
	border-bottom: 1px solid threedshadow !important;
}

toolbarseparator {
	border-right: 1px solid threedshadow !important;
	margin-right: 1px !important;
	margin-left: 1px !important;
}

statusbar {
	-moz-appearance: none !important;
	border-top: 1px solid threedshadow !important;
}

statusbarpanel {
	-moz-appearance: none !important;
	border-right: 1px solid threedshadow !important;

	margin-top: 2px !important;
	margin-bottom: 0 !important;
}

フラットメニュー:

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

menuseparator {
	background-image: none !important;

	-moz-border-top-colors: none !important;
	-moz-border-right-colors: none !important;
	-moz-border-bottom-colors: none !important;
	-moz-border-left-colors: none !important;

	border: none !important;
}

menuseparator {
	border-bottom: 1px solid threedshadow !important;
}