Unable to connect to websites which donot start with www
How do I prevent Firefox to use: www.foo.bar instead of foo.bar? suggests setting browser.fixup.alternate.enabled to disabled to be a working solution

Steps Involved Source


Turning off Domain Guessing


In the address bar, type about:config and press Enter
The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page. (There's no warranty with Mozilla so I suppose this is to just a warning joke)
In the Search field, type fixup.Double-click the browser.fixup.alternate.enabled preference to set its value to false. 

Changing the prefix (to www )


In the about:config page, double-click the browser.fixup.alternate.prefix 
In the pop up menu enter www.  ( if that doesn't help try www without . )



This is perhaps what was suggested in comment