The D.C. government tentatively has agreed to a contract with the city's firefighters' union that would give them an immediate 6 percent pay increase, retroactive to Dec. 1, 1981, followed by another 7 percent increase in October, Donald Weinberg, the city's chief labor negotiator, said yesterday.

The agreement closely resembles the contract negotiated with city police in November. That contract gives policemen the same wage increase, plus a 7 to 9 percent raise in 1983 and a new dental and optical plan, the first time such benefits have been granted to either police or firemen. Current base pay for firefighters ranges from $16,356 to $23,553 after 16 years of experience.

The International Association of Firefighters represents 1,240 firefighters from private through captain.

Weinberg said the agreement was reached at 5:30 a.m. Thursday, following an all-night negotiating session that ended 10 months of talks.

Among the issues negotiated, Weinberg said, was a new reduction-in-force or RIF procedure--"not that we're anticipating any lay-offs." Also, a new promotion procedure was approved that is expected to make it easier for blacks and other minorities to get promotions, according to Weinberg.

The agreement must be approved by Mayor Barry and the D.C. City Council and must be ratified by the firefighters.