The Federal Trade Commission rebuffed a request to launch an antitrust probe of Microsoft Corp. by a group of senators who said the Justice Department had gone soft in its long investigation of the software giant. FTC Chairman Robert Pitofsky, in a letter to four lawmakers, said an FTC probe "could involve a substantial duplication of effort as well as raise serious concerns about fairness to the targets and potential witnesses." The decision was a setback for Netscape Communications Corp. In 1994, the Justice Department negotiated a settlement over charges that Microsoft thwarted competition through its software licensing terms with computer manufacturers. Since that time, Mountain View, Calif.-based Netscape has charged that Microsoft gives discounts on Windows 95 operating systems to manufacturers who agree to feature Microsoft's Internet browser on a computer setup screen. Netscape also has complained that Microsoft is bundling some Internet browser and World Wide Web software with Microsoft's operating system products. In June, Sen. Conrad R. Burns (R-Mont.) and other lawmakers asked the FTC to investigate Microsoft. Shares of Microsoft, based in Redmond, Wash., rose $2.94 to close at $139.94 on Nasdaq.