changed select:-moz-dropdown-list back to collapse from hidden, this works better in more cases

r=kmcclusk

layout/html/document/src/html.css

@@ -1484,7 +1484,7 @@ sourcetext { /* XXX should not be in HTML namespace */
 
 /* combobox dropdown list hidden */
 select:-moz-dropdown-list {
-  visibility:hidden;
+  visibility:collapse;
 }
 
 /* combobox dropdown list visible */

layout/style/html.css

@@ -1484,7 +1484,7 @@ sourcetext { /* XXX should not be in HTML namespace */
 
 /* combobox dropdown list hidden */
 select:-moz-dropdown-list {
-  visibility:hidden;
+  visibility:collapse;
 }
 
 /* combobox dropdown list visible */