Firefox and kmail

Hello, When I clik on a Web link in Kmail, I receive this message : "you must close Firefox to be linked to this web address". (Firefox is already open). I go to Trinity Control Center : Components of TDE => Default applications "Web navigator" I erase the information "firefox" and put it again. It works, Firefox accepts to open, even if it is already open. After some days, Firefox refuses again, and same operation above described... Bug ? Thanks, André