The Firefoxers have been "improving" Firefox. One "improvement" which has been causing trouble since version 3.5 is "network prefetching". This software glitch tried to think ahead and go out and open websites before you need them. Unfortunately, it doesn't work right, and you draw a lot of "server not found" error screens. PITA.
There is a fix. You can turn network prefetching off and the bug goes away. I just did it, and it works like magic.
Just add a boolean variable named network.dns.disablePrefetch, with value "true" to firefox's config file. This file is named prefs.js and lives way down in Documents and Settings on the C drive. In principle you could merely edit this file with a text editor, but there is an easier way. From within Firefox go to pseudo-website about:config. Just put "about:config" on the address line as if it were a website you wanted to visit. Firefox will open prefs.js and display it in ASCII, in alphabetical order. Right click on a blank spot and Firefox will allow you to change values or add a new variable. Select "new" and enter network.dns.disablePrefetch as the name of the value. Assign it a type of Boolean, and a value of true. Fixes the problem.
Why has this bug persisted from Firefox 3.5 and into 3.6? The Firefox software people claim the problem comes from bugs in network routers and not Firefox. They may be technically correct, but that doesn't help us users much.