diff --git a/defaults/preferences/treestyletab.js b/defaults/preferences/treestyletab.js
index 90352227..0aef0d39 100644
--- a/defaults/preferences/treestyletab.js
+++ b/defaults/preferences/treestyletab.js
@@ -68,7 +68,7 @@ pref("extensions.treestyletab.tabbar.invertScrollbar", true);
  * This option works only for vertical tab bar.
  */
 pref("extensions.treestyletab.tabbar.narrowScrollbar", true);
-pref("extensions.treestyletab.tabbar.narrowScrollbar.size", "10px");
+pref("extensions.treestyletab.tabbar.narrowScrollbar.width", 10);
 
 /**
  * On some environments (ex. GNOME3 on Linux), "narrow scrollbar" cannot get
diff --git a/modules/base.js b/modules/base.js
index 3d20cd5e..c4e3a598 100644
--- a/modules/base.js
+++ b/modules/base.js
@@ -2276,7 +2276,7 @@ var TreeStyleTabBase = inherit(TreeStyleTabConstants, {
 			case 'extensions.treestyletab.tabbar.scrollToNewTab.mode':
 				return this.scrollToNewTabMode = value;
 
-			case 'extensions.treestyletab.tabbar.narrowScrollbar.size':
+			case 'extensions.treestyletab.tabbar.narrowScrollbar.width':
 				return utils.updateNarrowScrollbarStyle();
 
 			case 'browser.tabs.animate':
diff --git a/modules/utils.js b/modules/utils.js
index ff2f92f5..1085af29 100644
--- a/modules/utils.js
+++ b/modules/utils.js
@@ -519,15 +519,15 @@ var TreeStyleTabUtils = {
 			'  .tabbrowser-arrowscrollbox' +
 			'  > scrollbox' +
 			'  > scrollbar[orient="vertical"] * {' +
-			'  max-width: %SIZE%;' +
-			'  min-width: %SIZE%;' +
+			'  max-width: %SIZE%px;' +
+			'  min-width: %SIZE%px;' +
 			'}' +
 
 			'tabs.tabbrowser-tabs[%MODE%="vertical"][%NARROW%="true"]' +
 			'  .tabbrowser-arrowscrollbox' +
 			'  > scrollbox' +
 			'  > scrollbar[orient="vertical"] {' +
-			'  font-size: %SIZE%;' +
+			'  font-size: %SIZE%px;' +
 			'}' +
 
 			'tabs.tabbrowser-tabs[%MODE%="vertical"][%NARROW%="true"]' +
@@ -546,7 +546,7 @@ var TreeStyleTabUtils = {
 						this.kOVERRIDE_SYSTEM_SCROLLBAR_APPEARANCE : '' )
 				.replace(/%MODE%/g, this.kMODE)
 				.replace(/%NARROW%/g, this.kNARROW_SCROLLBAR)
-				.replace(/%SIZE%/g, utils.getTreePref('tabbar.narrowScrollbar.size'))
+				.replace(/%SIZE%/g, utils.getTreePref('tabbar.narrowScrollbar.width'))
 			);
 		this.lastAgentSheetForNarrowScrollbar = this.makeURIFromSpec(style);
 		SSS.loadAndRegisterSheet(this.lastAgentSheetForNarrowScrollbar, SSS.AGENT_SHEET);