Monday, November 19th 2007, 4:00 AM A startling home video captured the image of a firefighter tumbling out of a ladder bucket to the concrete 30 feet below Sunday. Brian Smith, an 11-year FDNY veteran, was three floors above the street, battling a Bronx apartment building fire, when a hatch apparently gave way. As smoke billows and sirens wail, Smith, 43, assigned to Ladder Co. 44, suddenly lurches out of the cherry picker and falls out of sight. "He just fell down and hit the [ground]. He hit his head," said Darnell Sanabria, 21, who saw the drama on Shakespeare Ave. in High Bridge. "Everybody screamed, 'Oh, my God!'" Smith was taken to St. Barnabas Hospital, where he was listed in stable condition last night. Colleagues said he was conscious and alert. An FDNY safety officer told the Daily News that their cherry picker buckets have doors that are secured with a latch. A preliminary check revealed the latch was operating properly, suggesting Smith may have mistakenly believed the door was secured when he leaned against it.