A Falls Church man who worked as a federal informant on terrorism set himself on fire in front of the White House yesterday, hours after announcing his suicide attempt and citing his growing despondency over how the FBI managed his case. Mohamed Alanssi, 52, approached the northwest guardhouse on Pennsylvania Avenue about 2:05 p.m. and asked the security detail to deliver a note to President Bush. When uniformed Secret Service officers turned him away, he stepped about 15 feet from the guard post and used a lighter to ignite his jacket, according to the U.S. Park Police.