The D.C. government and the union representing 1,200 firefighters signed a new three-year contract yesterday that provides a bonus and pay increases of 9.5 percent, as well as major changes in staffing and work schedules that could save the city millions of dollars a year.

Mayor Marion Barry, Fire Chief Theodore R. Coleman and Thomas Tippett, president of Local 36 of the International Association of Firefighters, announced the agreement during a ceremony at Engine Company No. 6 at 1300 New Jersey Avenue NW.

For the first time, fire truck companies will be operated with a crew of five instead of six, a money-saving step that the firefighters have fought in the past. A study will be done in the second year of the contract to determine whether further cutbacks are feasible.

City officials said the reductions will be accomplished without layoffs and without jeopardizing safety and firefighting capability.

The District also agreed to experiment for 18 months with new work shifts for firefighters of 24 hours on duty followed by 48 hours off, instead of the current unpopular system of a 10-hour work day followed by a 14-hour shift at night.

Fire departments in other large cities have cut down on sick leave and improved morale among firefighters by going to the so called "24-48 pattern," according to city and union officials.

D.C. firefighters have been working without a contract since Sept. 30. The mayor described the nine months of bargaining that led to the agreement as "one of the toughest set of labor negotiations we have experienced thus far."

"This agreement is fair to our employes and will enable us to continue to provide the highest quality of services to our residents," Barry said.

Tippett said the experimental changes in work shifts "will show some positive signs to all that the city is willing to look at the firefighters' problems and address them."

"We're hoping that as a result of finally having a contract that this is going to have a positive impact on morale," said Tippett, whose union in the past has sharply criticized Chief Coleman's leadership and policies.

Under the new agreement, firefighters will receive a one-time lump sum payment this year of 3 percent of their pay. The bonuses will range from about $600 for new firefighters to $1,200 for captains.

Firefighters then will receive a 4 percent pay increase next October, another 4 percent increase in October 1986, and a 1.5 percent increase in April 1987. The three-year contract expires Sept. 30, 1987.

The annual base salary for a beginning firefighter will rise from $19,850 to $21,780 under the new contract. Base pay for sergeants will increase from $26,959 to $29,579, pay for lieutenants will increase from $31,161 to $34,189 and pay for captains will go from $36,918 to $40,507.

The contract also includes a continuation of optical coverage and improvement in dental health insurance, one additional leave day and a new uniform and training-materials allowance of $325 the first year and $400 in each of the subsequent years.

The package will cost the District an additional $9 million during the next three years. However, the city could offset $5 million to $6 million of that cost through the new staffing plan and other changes, according to Donald Weinberg, the city's chief negotiator.