Browse to 'about:config'.
Type 'browser.newtab.url' and amend value to 'about:blank'.
If the value is changed to 'about:http://www.google.co.uk' a new tab will open with Google, but with the cursor in the address bar, not the search box.
If the value is deleted, then the Firefox add-on 'NewTabURL 2.2.3' can be set to go to Google with the cursor already in the search box. Or of course any other URL can be used.



