I'm not very adept at using command prompts
Also, I don't know what you mean by renaming the file extension with Windows Explorer. Right now, Roboform-Palemoon.zip is located on the Desktop. As I previously mentioned, I tried to rename the file to
Roboform-Palemoon.xpi by deleting "zip" and typing "xpi" in its place, but it resulted in a corrupted, unreadable file. I really don't have a clue when it involves file management.