Windows 7 Professional - fully patched
Norton Internet Security 17.7.0 12
Firfox 3.6.8
After most (maybe all) downloads within Firefox, the virus scan takes an unusually long time (30 seconds or more) and I don't get the normal notification of the scan results. Upon closing Firefox the file is deleted from my download folder and can't be found anywhere on the computer. If I move the file before closing Firefox the file is not deleted.
Several other people have reported this same problem in the Mozilla support forum. It appears there are two ways to solve it without abandoning Norton or Firefox: either disable automatic malware scans after downloads in Firefox (yikes), or disable Norton's "Microsoft Office Automatic Scan" (less yikes).
Norton's MOAS is appears to be the culprit. I only started having the problem today after enabling MOAS.