Reload web page(s) and bypass the cache.

• Press and hold Shift and left-click the Reload button.
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Cmd + Shift + R" (MAC)

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove the Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

I did what you suggested asnd Cleared the Cache and Removed all Cokkies, but nothing has changed. It still stops at 404 Not Found. I didn't understand the bit about bypass the cache as I can't see anything that says Reload.

Thanks anyway

This is Yahoo's troubleshooting site: http://help.yahoo.com/l/us/yahoo/mail/yahoomail/

Firefox 4 uses a combined Stop/Reload/Go button that is positioned at the right end of the location bar.
During the page load process it shows as a Stop button and after the loading has finished the button is changed to a Reload button.
If you type in the location bar then that button becomes a Go button.
Middle-click the Reload button to duplicate the current tab to a new tab.

Are you using a bookmark to go to that page or are you typing the address (URL) in the location bar?