--- a/devtools/client/themes/webconsole.css
+++ b/devtools/client/themes/webconsole.css
@@ -18,16 +18,20 @@ a {
   cursor: pointer;
   text-decoration: underline;
 }
 
 /* Workaround for Bug 575675 - FindChildWithRules aRelevantLinkVisited
  * assertion when loading HTML page with links in XUL iframe */
 *:visited { }
 
+* {
+  box-sizing: border-box;
+}
+
 .message {
   display: flex;
   padding: 0 7px;
   width: 100%;
   box-sizing: border-box;
 }
 
 .message > .prefix,