you out. http://3d2f.com/programs/54-023-zipzag-download.shtml
Says it handles zip and xpi compression and "Some of the features include extract, add, delete, self-extract (sfx), unsfx, convert, batch add, batch extract, merge, run, install, checkout, view, test, virus scan, repair. Has a build-in viewer that works with most formats."
This link indicates that although Mozilla expects an xpi file extension the file format of the compressed extension file is actually zip so renaming the file from whatever.zip to whatever.xpi should be simple. If you are having trouble renaming the file try doing it with the command prompt.
If for instance, the file is located at C:\downloads\zipfile\whatever.zip you simply open a command prompt and type the following:
ren "C:\downloads\zipfile\whatever.zip" whatever.xpi (the quotes are only needed if the actual path has spaces in it but I placed them in this example so you could see how they are used. Since your actual path and file names are different edit the command to reflect actual path and filenames.)
You might also try renaming with Windows Explorer as you have been doing but enclose the file name in quotes. Thus you start with whatever.zip and rename it to "whatever.xpi" and those quotes you need to include will not actually be included in the renaming so the file name ends up as whatever.xpi