The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET http://www.facebook.com/?q=.
Reason: Error reading from remote server
Apache Server at www.facebook.com Port 3128
Clear the cache and the cookies from sites that cause problems.
"Clear the Cache":
"Remove Cookies" from sites causing problems:
Check the connection settings.
If you do not need to use a proxy to connect to internet then try to select "No Proxy" if the "Use the system proxy settings" setting or one of the others doesn't work properly.
See "Firefox connection settings":