--- a/toolkit/modules/GMPInstallManager.jsm
+++ b/toolkit/modules/GMPInstallManager.jsm
@@ -416,16 +416,19 @@ GMPExtractor.prototype = {
         outFile.appendRelativePath(entry);
 
         // Make sure the directory hierarchy exists
         if(!outFile.parent.exists()) {
           outFile.parent.create(Ci.nsIFile.DIRECTORY_TYPE, parseInt("0755", 8));
         }
         zipReader.extract(entry, outFile);
         extractedPaths.push(outFile.path);
+        // Ensure files are writable and executable. Otherwise we may be unable to
+        // execute or uninstall them.
+        outFile.permissions |= parseInt("0700", 8);
         log.info(entry + " was successfully extracted to: " +
             outFile.path);
       });
       zipReader.close();
       if (!GMPInstallManager.overrideLeaveDownloadedZip) {
         zipFile.remove(false);
       }