diff --git a/browser/themes/shared/devtools/projecteditor/projecteditor.css b/browser/themes/shared/devtools/projecteditor/projecteditor.css
index 1d11f603bbe8..9878b0e80677 100644
--- a/browser/themes/shared/devtools/projecteditor/projecteditor.css
+++ b/browser/themes/shared/devtools/projecteditor/projecteditor.css
@@ -27,7 +27,7 @@
 
 .sources-tree {
   overflow:auto;
-  -moz-user-focus: normal;
+  -moz-user-focus: true;
 }
 
 .sources-tree input {