MARTIN COUNTY, Fla. - A 10-year-old Florida girl was beginning her first day of online school when her mother was shot to death by her ex-boyfriend at their home, investigators said Tuesday.Donald Williams, 27, shot and killed 32-year-old Maribel Rosado Morales in front of her four children and their two cousins at her Indiantown home, Martin County Sheriff William Snyder told reporters at a press conference, which was recorded by WPEC-TV.The shooting unfolded during an argument after Williams entered the home around 8 a.m.