As a teen-ager, Danny Selby hung around the Rockville fire station washing fire engines, cleaning hoses and fetching sandwiches and coffee for the seasoned firefighters.

After he graduated from Rockville High School in 1980, the attraction of fighting fires lingered. Even though he worked days as a lumber salesman, he spent his nights and weekends as a volunteer firefighter in Rockville, and for several years he had been living at Fire Station 3, on Hungerford Drive, sleeping at the station every night.

Yesterday, the man described by colleagues as a happy-go-lucky, quiet "kid" was in critical condition in the intensive care unit of Suburban Hospital in Bethesda. While fighting a fire in Potomac Sunday night, he fell through a roof and was pulled out, but then he fell 15 feet to the ground. Selby, 23, underwent surgery yesterday afternoon to remove a blood clot from his brain. Rockville Fire Department officials said he received first- and second-degree burns as well.

Selby's colleague, Michael Lavelle, 33, who fights fires for Rockville and Gaithersburg stations, pulled Selby from the fire, but he then lost his balance and the two fell to the ground. Lavelle received a broken left kneecap and was treated and released from Suburban.

Selby's injuries, his friends and fellow firefighters said, have brought home the precarious nature of their jobs. The mood was somber yesterday at Selby's Rockville station, where most of the firefighters are volunteers, although many of them also are paid Montgomery County employes at the Gaithersburg station.

"This has hit the troops pretty hard," said Leslie Adams, assistant chief of the Rockville Volunteer Fire Department, who has called a meeting for Thursday so that the firefighters can "get their feelings out" about the accident.

Selby's "prognosis is good, and everybody here is pulling for him," said Neil Shorb, 27. "He's a fighter, and I know that stubborn streak will get him through this."

Shorb was on the rooftop about nine feet from Selby and Lavelle when the accident occurred. He said he felt helpless at the time of the accident.

The three men had climbed the roof of the house at 2305 Stratton Dr. to cut through the roof and ceiling to let the heat and smoke escape, a common practice that makes it easier for firefighters to enter a house and douse the blaze.

The fire began in the kitchen of the two-story house, caused by a pan of grease heating on a stove that was left unattended, fire officials said. It caused about $270,000 in damage.

As the three men crawled along the roof toward a ladder that leaned against the house, the roof suddenly gave way and Selby fell in. "He was engulfed in a ball of flame," said Larry Gaddis, who fights fires for Rockville and Gaithersburg stations and who was watching what happened from the ground.

Lavelle grabbed Selby by his coat and managed to pull him out from under the roof, but the two men lost their balance, rolled across the roof and tumbled to the sidewalk below.

"When they hit the ground it was like a sack of bricks. I heard a 'bam' and then silence," Gaddis said.

Gaddis said Lavelle held onto Selby as the two men fell.

"I was not going to let go," explained Lavelle, a short, muscular man with steel blue eyes and curly hair.

"He was my partner. We went in there together and we were going to come out together."

Lavelle, who has been a firefighter for 12 years, said that occasionally he asks himself, "Why do I do this? If people run out of fires, why do we idiots run into them?"

In the next breath, however, he conceded: "I wouldn't do anything else."