The latest update was Gingerbread(2.3). The baseband version was: T959VUVKJ6. I did find a copy available for download, Since, I am not familiar with the site, please use your antivirus software to be sure that the file is clean of any viruses. You can also use the firmware baseband firmware to search online as well.
Alternatively, if the current ROM you are using is causing issues, you might also want to review other ROMs available for your phone on the XDA-Developers wiki that may resolve the specific issues you are having. There are many that are based off the stock firmware.