Upgrading to FF3.5 may lead to DNS leaks when you are using secure proxies, even if it didn't leak before. DNS leaks are irrelevant to the websites you visited. But you ISP's DNS server will know the URL's you want to visit. The details. FF3.5 made some internal changes regarding the timing of the DNS requests and the http requests. They are themselves not bugs. If you enter your proxy into FF manually, and set network.proxy.socks_remote_dns to True, there will not be leaks. Proxy switching add-ons such as Foxy Proxy, switch proxy at every http request, and try to preserve the default proxy of your browser in between switches. DNS leaks occur because the requests are not synchronized with the http requests, selecting the wrong proxy for DNS. There may be options in Foxy to correct this with sock proxies. But fast and popular secure proxies like Ultrasurf isn't a sock proxy. The safer way to avoid leaks is to use a simple proxy switch add-on like Multiproxy Switch. The selected proxy is the proxy in use at all times.