Constant Firefox Errors.

So when I'm not being told that the 'Address Not Found' I'm getting stupid page errors like this;

http://uploadpicz.com/images/3D78HQW.jpg

How to fix?

hate to state the obvious but...

have you cleared everything?
and reinstalled it?
and restarted your computer? and modem?

Brand new PC, Just downloaded FF. Errors also happened on my old pc.

Using different OS, everything is different.