Striking firefighters were to return to work Thursday after Mayor Jane Byrne agreed today to a 24-hour deadline for completing a union contract.

Under the agreement, read late tonight resume at 11 a.m. Thursday simultaneous with the return to work of the 4,300 firefighters who walked off their jobs a week ago.

As part of the package deal hammered out over a two-day period, the firefighters union agreed not to penalize members, who crossed picket lines, and the city promised not to take "disciplinary or retaliatory acts" against strikers so long as they were not involved in "criminal or quasi-criminal" activity.

Circuit Judge John Hechinger, in whose courtroom the agreement was reached said: "I can only say that I think that, finally, cool heads have prevailed."

But there was little indication that the two sides were any closer to an agreement that they were when the walkout began. The main issue dividing them include manning of fire equipment and the size of the firefighters bargaining unit.

A frist contract for city firemen was one of Byrne's campaign promises when she ran against the city's Democratic machine a year ago.

During the strike, Byrne said that the men who walked out would "never" be allowed to return to their jobs. She also began hiring replacements whom the union promptly labeled "strike-breakers."

The city's powerful union leaders had met with Bryne until early this morning trying to convince her to reopen talks with the firefighters. A citywide sympathy srike was threatened if she failed to heed the union leader' advice.

Byrne dismissed these reports, and said top labor leaders had offered their assistance in ending the firefighters' strike.

Under the agreement, the two sides have 24 hours to complete the talks, though extension are possible. The strikers have been fined $40,000 a day for contempt of Hechinger's ruling that their strike is illegal, but so far they have refused to pay. As part of the order, Hechinger waived all but two days of the fine.