Recently I have been getting random Server Not Found pages in Firefox (v18.02) on my Windows 8 laptop. Clicking the Try again button on this page takes me to the page I want, but this is annoying.
Update: The fix below turned out to be temporary. I had to open a command prompt and enter ipconfig /flushdns to resolve the issue.
The solution for me was to change the proxy setting in Firefox to Auto-detect proxy settings for this network as follows.
- Click the Firefox button
- Click Options on the drop-down menu and then click Options on the pop-out menu
- Click the Advanced button and then click the Network tab
- Click the Settings button in the Connection section of the window (Configure how Firefox connects to the internet)
- Click the Auto-detect proxy settings for this network radio button and then click OK to exit the menus.
- Restart Firefox.
Michael Lane's Technology Blog 
This is an old post on a wordpress blog so I don’t know if you’ll ever see this, but dude I love you. I have been having this problem for the longest time, and for the life of me could not find a solution. It frustrated me to no end having some sites work one day, then not work the next.
Glad it worked for you Matthew! 🙂