--- a/devtools/client/themes/common.css
+++ b/devtools/client/themes/common.css
@@ -26,20 +26,16 @@
 :root.theme-firebug {
   --proportional-font-family: Lucida Grande, Tahoma, sans-serif;
 }
 
 .devtools-monospace {
   font-family: var(--monospace-font-family);
 }
 
-:root[platform="linux"] .devtools-monospace {
-  font-size: 80%;
-}
-
 /* Override wrong system font from forms.css */
 html|button, html|select {
   font: message-box;
 }
 
 /* Autocomplete Popup */
 
 .devtools-autocomplete-popup {

--- a/devtools/client/themes/rules.css
+++ b/devtools/client/themes/rules.css
@@ -14,21 +14,16 @@
 .theme-firebug {
   --rule-highlight-background-color: #ffee99;
   --rule-property-name: darkgreen;
   --rule-property-value: darkblue;
 }
 
 /* Rule View Tabpanel */
 
-.theme-firebug .ruleview {
-  font-family: monospace;
-  font-size: 11px;
-}
-
 #sidebar-panel-ruleview {
   margin: 0;
   display: flex;
   flex-direction: column;
   width: 100%;
   height: 100%;
   /* Override the min-width from .inspector-tabpanel, as the rule panel can support small
      widths */