Possible Duplicate:
Firefox 4, turn off “switch to tab” in URL bar?
When I type a URL in the address bar, if the same url is opened in another tab, instead of auto-fill option it prompt only for switching to that tab. Is there any way to disable this feature?
Note: this looks like a new feature in Firefox 4.0.

-ProfileManagerswitch to see if one of your add-ons is responsible. – Daniel Beck May 5 '11 at 7:37