Blazing Comedy: The comedy during two special shows at the Laugh Factory tonight will be for a serious cause: a benefit for the Los Angeles Firemen's Relief Assn. in care of the Widows and Orphans Fund. Special tribute will be paid to firefighter Scott Miller, who was shot in the face during the Los Angeles riots. . . . And on Monday, comic Elayne Boosler will present "A Hot Time in the Old Town Tonight," a free concert for Los Angeles County firefighters who fought the blazes during the city's recent strife.