diff --git a/layout/style/res/ua.css b/layout/style/res/ua.css
index ab51f67c53..bf32e66c56 100644
--- a/layout/style/res/ua.css
+++ b/layout/style/res/ua.css
@@ -320,6 +320,8 @@
   position: fixed;
   top: 0; left: 0;
   right: 0; bottom: 0;
+  /* This prevents undesired interactions with the selection code. */
+  -moz-user-select: none;
 }
 
 *|*:-moz-full-screen:not(:root)::backdrop {