This morning when I turned on my computer, Firefox greeted me by saying "Firefox is configured to use a proxy that is refusing connections." However, my proxy server was turned on and seemed to be functioning correctly - I could ssh into it quite fine, so no network issues. I rebooted, but still no help. My server was running Ubuntu 8.10 (Intrepid) Server Edition with a squid proxy server.
Turns out, by examing the /var/log/squid/cache.log file, I discovered the problem was no free disk space. After freeing up some space on / and restarting squid (/etc/init.d/squid restart), proxy was up and running again.