Janesville firefighters are investigating a residential blaze on the southwest side of the city.  Battalion Chief Chris Lukas says firefighters responded to the 1500 block of South River Road just after 2:00 a.m. Friday.  The first crew to arrive found heavy flames on the first floor.  Three adults did escape, but two suffered minor injuries, including burns, and needed to be taken to the hospital for treatment.  Lukas says damage is extensive to the first floor of the home and the residents are displaced.  The preliminary investigation suggests the cause may be related to the improper use of smoking materials.