--- a/browser/base/content/test/static/browser_parsable_css.js
+++ b/browser/base/content/test/static/browser_parsable_css.js
@@ -67,16 +67,21 @@ let whitelist = [
    isFromDevTools: true},
 ];
 
 if (!Services.prefs.getBoolPref("full-screen-api.unprefix.enabled")) {
   whitelist.push({
     sourceName: /(?:res|gre-resources)\/(ua|html)\.css$/i,
     errorMessage: /Unknown pseudo-class .*\bfullscreen\b/i,
     isFromDevTools: false
+  }, {
+    // PDFjs is futureproofing its pseudoselectors, and those rules are dropped.
+    sourceName: /web\/viewer\.css$/i,
+    errorMessage: /Unknown pseudo-class .*\bfullscreen\b/i,
+    isFromDevTools: false
   });
 }
 
 let propNameWhitelist = [
   // These are CSS custom properties that we found a definition of but
   // no reference to.
   // Bug 1441837
   {propName: "--in-content-category-text-active",