User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:2.0) Gecko/20100101 Firefox/4.0 Build Identifier: Mozilla/5.0 (Windows NT 5.1; rv:2.0) Gecko/20100101 Firefox/4.0 If i configure proxy to use auto-configure (.pac) file, every time open a new link url, the UI will hanging for a long time. I tested ervery line in pac file, found the culprit is the "inInNet" function. " if (isInNet(host, "192.168.0.0", "255.255.0.0") ||isInNet(host, "172.16.0.0", "255.240.0.0") ||isInNet(host, "10.0.0.0", "255.0.0.0") ||isInNet(host, "127.0.0.0", "255.255.255.0") ) return "DIRECT"; " Reproducible: Always Steps to Reproduce: 1.Create a demo.pac file which include "isInNet" function call. 2.Set the proxy to use url which reference demo.pac file. 3.Open a webpage and then open several link pages. Actual Results: The firefox 4 UI hang for a long time. Expected Results: The UI will keep responsiveness. The content in demo.pac file: " if (isInNet(host, "192.168.0.0", "255.255.0.0") ||isInNet(host, "172.16.0.0", "255.240.0.0") ||isInNet(host, "10.0.0.0", "255.0.0.0") ||isInNet(host, "127.0.0.0", "255.255.255.0") ) return "DIRECT"; "