diff --git a/layout/style/nsCSSKeywordList.h b/layout/style/nsCSSKeywordList.h
index 5c0057aa76..d389f4fe35 100644
--- a/layout/style/nsCSSKeywordList.h
+++ b/layout/style/nsCSSKeywordList.h
@@ -72,6 +72,7 @@ CSS_KEY(-moz-grid, _moz_grid)
 CSS_KEY(-moz-groupbox, _moz_groupbox)
 CSS_KEY(-moz-gtk-info-bar, _moz_gtk_info_bar)
 CSS_KEY(-moz-gtk-info-bar-text, _moz_gtk_info_bar_text)
+CSS_KEY(-moz-hidden-unscrollable, _moz_hidden_unscrollable)
 CSS_KEY(-moz-hyperlinktext, _moz_hyperlinktext)
 CSS_KEY(-moz-html-cellhighlight, _moz_html_cellhighlight)
 CSS_KEY(-moz-html-cellhighlighttext, _moz_html_cellhighlighttext)
diff --git a/layout/style/nsCSSProps.cpp b/layout/style/nsCSSProps.cpp
index 1e798504b4..b83005dd6a 100644
--- a/layout/style/nsCSSProps.cpp
+++ b/layout/style/nsCSSProps.cpp
@@ -1876,6 +1876,7 @@ const KTableEntry nsCSSProps::kOverflowKTable[] = {
   { eCSSKeyword__moz_scrollbars_none, NS_STYLE_OVERFLOW_HIDDEN },
   { eCSSKeyword__moz_scrollbars_horizontal, NS_STYLE_OVERFLOW_SCROLLBARS_HORIZONTAL },
   { eCSSKeyword__moz_scrollbars_vertical, NS_STYLE_OVERFLOW_SCROLLBARS_VERTICAL },
+  { eCSSKeyword__moz_hidden_unscrollable, NS_STYLE_OVERFLOW_CLIP },
   { eCSSKeyword_UNKNOWN, -1 }
 };
 
@@ -1891,6 +1892,7 @@ const KTableEntry nsCSSProps::kOverflowSubKTable[] = {
   { eCSSKeyword_hidden, NS_STYLE_OVERFLOW_HIDDEN },
   { eCSSKeyword_scroll, NS_STYLE_OVERFLOW_SCROLL },
   { eCSSKeyword_clip, NS_STYLE_OVERFLOW_CLIP },
+  { eCSSKeyword__moz_hidden_unscrollable, NS_STYLE_OVERFLOW_CLIP },
   { eCSSKeyword_UNKNOWN, -1 }
 };