First thing, try going to about:config and change this setting:
network.proxy.socks_version=4
if you have a proxy installed. If it doesn't work, or you do not use a proxy, then change it back to 5, then do these:
network.http.keep-alive=false
network.http.proxy.keep-alive=false
If it doesn't work, change them back to true.
You may also want to import your Internet Explorer settings if you did not do so previously (File > Import... > Microsoft Internet Explorer) and seeing if that makes a difference.*