drragostea: Thank you for your response. Actually, I managed to get it done by installing the Portable.Apps version of Firefox in a folder (Firefox (2)) in my 'C' drive, running Spybot (which recognized and immunized the portable version), then cutting the folder and pasting it into my USB device. It works OK, although (if anyone else wants to try it) one must be aware that this version of Firefox appears a bit slow and unresponsive (compared to the earlier version available as a U3 application) - so one mustn't think that it is the addition of Spybot immunization that slows it down.
PS: Nikkormats (1970 & 1972), 28mm f./3.5, 50mm f./1.4, 55mm f./3.5 Micro, & 200mm f/4 Auto-Nikkors.