diff --git a/devtools/server/actors/highlighters.css b/devtools/server/actors/highlighters.css
index de41b2a601ea..480bf0ec3df0 100644
--- a/devtools/server/actors/highlighters.css
+++ b/devtools/server/actors/highlighters.css
@@ -14,7 +14,7 @@
 */
 
 :-moz-native-anonymous .highlighter-container {
-  position: absolute;
+  position: fixed;
   width: 100%;
   height: 100%;
   /* The container for all highlighters doesn't react to pointer-events by
diff --git a/layout/style/res/ua.css b/layout/style/res/ua.css
index 9080f9da2f0b..1c22076a208f 100644
--- a/layout/style/res/ua.css
+++ b/layout/style/res/ua.css
@@ -431,12 +431,12 @@ div:-moz-native-anonymous.moz-accessiblecaret.none {
   }
 }
 
-/* Custom content container in the CanvasFrame, fixed positioned on top of
+/* Custom content container in the CanvasFrame, positioned on top of everything
    everything else, not reacting to pointer events. */
 div:-moz-native-anonymous.moz-custom-content-container {
   pointer-events: none;
   -moz-top-layer: top;
-  position: fixed;
+  position: absolute;
   top: 0;
   left: 0;
   width: 100%;