Updates firefox logo in unsupported page.

css/unsupported_browser.css

     height: 79px;
     background-image: url('/images/chromium.png');
 }
-#firefox-nightly_logo
+#firefox_logo
 {
     width: 73px;
     height: 79px;
-    background-image: url('/images/firefox-nightly.png');
+    background-image: url('/images/firefox.png');
 }
 
 #opera_logo

unsupported_browser.html

         <div class="browser_wrapper">
             Firefox
             <div class="browser">
-                <div class="logo" id="firefox"></div>
+                <div class="logo" id="firefox_logo"></div>
                 <a href="http://www.getfirefox.com/"><div class="button">DOWNLOAD</div></a>
             </div>
         </div>