A possible cause is security software (firewall) that blocks or restricts Firefox without informing you about that, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox.

See Server Not Found - Troubleshoot connection problems and Configure firewalls so that Firefox can access the Internet and http://kb.mozillazine.org/Firewalls

That worked! Thanks ever so much best regards Richard

You're welcome

Hi,Guys well where do i begin i have had the same problem for the last 8 days it was driving me mad,I tried every thing I trawled the internet for an answere,have been awake now for 2 days without sleep tried all the suggestions on the net i could find,uninstalled firefox twice,removed antivirus,removed firewall,tried recovery twice ,and finaly after a complete system restore,(I have an external harddrive so i transfered every thing of importance onto that) and took my PC back to its factory settings,thinking thats bound to solve it rebooted put back everything i needed and still the same problem in the end I could only access a page for a short time, even resetting the router was of no help,I was in dispare when i came upon a small comment on a blog..........WHAT WAS IT.........it was....... ADOBE READER X.....so i uninstalled it,dowloaded ....adobe reader 9....it took about 10 minutes to get me up and running....its 07.20 and im off to bed...but i had to share my discovery....it may help somebody in a similar plight....GOOD LUCK.........happy surfing... :0)

My solution was to go back to Adobe 9 Reader and reload Firefox 4 - everything works fine now!

I have the exact problem as you gonzo with Firefox 5 and OMG I can't believe the culprit was ADOBE X!!! I was checking Firewall, FF settings,reset router. Then I realised my other browsers were working so it was back to FF settings... Thanks for this

Ok.. Here's THE foolproof method of solving "Website can't be found", "Error loading page" and all other miscellaneous issues with this lame product.

1) Go into control panel

2) Click Programs

3) Scroll down to "Firefox"

4) Click "Remove Firefox"

5) Close Control Panel

6) Click the IExplorer icon which came with Windows.

Hope this helps.

Actually, dchristie, your solution didn't help at all. If I wanted to use IE, I wouldn't have installed Firefox. IE is a good browser, but doesn't have features Firefox has. Also, when Firefox crashes windows can kill it and you can restart it no problem. When IE crashes, since it is so tightly integrated into windows, windows itself becomes unstable and you have to reboot.

Also check your personal security firewall...McAfee blocked my Firefox so I did this from the McAfee info on my computer:

1 Open the Firewall settings page.

How?
                         1 On the Home Page, click Firewall.  
                         2 Click Settings.  

2 Click Program Permissions. 3 Select a program with Outgoing or Blocked access. 4 Click Edit.

To select a new program, click Add, then browse to and select the program you want to add permissions for. 

5 Under Access, click the arrow to pull down the menu, and select Full. 6 Click Apply.

I have uninstalled Adobe X and installed Adobe reader 9.5, this at least solved my problem of not being able see videos on the WWE website. However I still can't open Mozilla.org, I have tried uninstalling Firefox and then reinstalling it. Still no luck, I even (heaven forbid) tried to open it in Internet Explorer, however it still won't load. I can open any page I want except the Mozilla.org page and therefore can't download the latest version of Firefox. Any help on this would be greatly appreciated.

I have managed to download the latest firefox version from www.getfirefox.net.