--- a/devtools/client/themes/firebug-theme.css
+++ b/devtools/client/themes/firebug-theme.css
@@ -2,17 +2,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 @import url(resource://devtools/client/themes/variables.css);
 @import url(resource://devtools/client/themes/common.css);
 @import url(chrome://devtools/skin/light-theme.css);
 
-:root {
+:root.theme-firebug {
   font-size: 11px;
   font-family: var(--proportional-font-family);
 }
 
 /* CodeMirror Color Syntax */
 
 .theme-firebug .cm-keyword {color: BlueViolet; font-weight: bold;}
 .theme-firebug .cm-atom {color: #219;}