Will today's giddy economic boom give way to a brutal postholiday hangover? Last week the Dow Jones stock average slipped on confusing news of megamergers by some companies and thousands of layoffs at others. It then recovered slightly after the Department of Labor announced that the November unemployment rate dipped to 4.4 percent. But when it comes to jobs, the bad news may outweigh the good. Challenger, Gray and Christmas, a Chicago-based outplacement firm, estimates that 1998 may set the decade's record for layoffs. It reported that U.S. firms eliminated 574,629 jobs this year -- 32 percent more than in 1997 and within reach of the 1993 high of 615,185. Some economists argue that job gains will continue to offset job losses. But economic turmoil in Asia and Latin America may make things worse. "We are on the edge of deflation here," says Edward Yardeni, chief economist at Deutsche Bank Securities N.A. "Companies are under enormous pressure, and that means more mergers and layoffs." If that's true, the party will be over in 1999.