--- a/toolkit/system/windowsproxy/nsWindowsSystemProxySettings.cpp
+++ b/toolkit/system/windowsproxy/nsWindowsSystemProxySettings.cpp
@@ -213,16 +213,17 @@ nsWindowsSystemProxySettings::GetProxyFo
 
     if (MatchOverride(aHost)) {
         SetProxyResultDirect(aResult);
         return NS_OK;
     }
 
     NS_ConvertUTF16toUTF8 cbuf(buf);
 
+    NS_NAMED_LITERAL_CSTRING(kSocksPrefix, "socks=");
     nsAutoCString prefix;
     ToLowerCase(aScheme, prefix);
 
     prefix.Append('=');
 
     nsAutoCString specificProxy;
     nsAutoCString defaultProxy;
     nsAutoCString socksProxy;
@@ -244,19 +245,19 @@ nsWindowsSystemProxySettings::GetProxyFo
                 // (http://msdn.microsoft.com/en-us/library/aa383996%28VS.85%29.aspx)
                 defaultProxy = proxy;
             } else if (proxy.Find(prefix) == 0) {
                 // To list a proxy for a specific protocol, the string must
                 // follow the format "<protocol>=<protocol>://<proxy_name>".
                 // (http://msdn.microsoft.com/en-us/library/aa383996%28VS.85%29.aspx)
                 specificProxy = Substring(proxy, prefix.Length());
                 break;
-            } else if (proxy.Find("socks=") == 0) {
+            } else if (proxy.Find(kSocksPrefix) == 0) {
                 // SOCKS proxy.
-                socksProxy = Substring(proxy, 5); // "socks=" length.
+                socksProxy = Substring(proxy, kSocksPrefix.Length()); // "socks=" length.
             }
         }
 
         if (delimiter == end)
             break;
         start = ++delimiter;
     }