diff --git a/browser/extensions/pdfjs/content/PdfStreamConverter.jsm b/browser/extensions/pdfjs/content/PdfStreamConverter.jsm
index 3a26bcd2d3df..18b3985c3762 100644
--- a/browser/extensions/pdfjs/content/PdfStreamConverter.jsm
+++ b/browser/extensions/pdfjs/content/PdfStreamConverter.jsm
@@ -93,7 +93,7 @@ function getIntPref(pref, def) {
 
 function getStringPref(pref, def) {
   try {
-    return Services.prefs.getComplexValue(pref, Ci.nsISupportsString).data;
+    return Services.prefs.getStringPref(pref);
   } catch (ex) {
     return def;
   }
diff --git a/browser/extensions/pdfjs/content/PdfjsChromeUtils.jsm b/browser/extensions/pdfjs/content/PdfjsChromeUtils.jsm
index 33ff96656156..968e38ae1542 100644
--- a/browser/extensions/pdfjs/content/PdfjsChromeUtils.jsm
+++ b/browser/extensions/pdfjs/content/PdfjsChromeUtils.jsm
@@ -290,10 +290,7 @@ var PdfjsChromeUtils = {
 
   _setStringPref(aPrefName, aPrefValue) {
     this._ensurePreferenceAllowed(aPrefName);
-    let str = Cc["@mozilla.org/supports-string;1"]
-                .createInstance(Ci.nsISupportsString);
-    str.data = aPrefValue;
-    Services.prefs.setComplexValue(aPrefName, Ci.nsISupportsString, str);
+    Services.prefs.setStringPref(aPrefName, aPrefValue);
   },
 
   /*