Can't install IconPack in Portable Firefox: "Install script not found"

Hey All,

Try as I might, I've been unable to get any iconpack to install in Firefox Portable. I've tested the same iconpacks in "regular" firefox, and even firefox_thinstall, and they worked fine in those. When I install it, I get the following:

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Firefox could not install the file at

because: Install script not found
-204
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I've tried the following:
Install the xpi directly from the site: same error
Save xpi locally, then drag/drop into FFP window: same error

I even tried to copy the files to locations where they seemed to belong within the install, all to no avail, and I couldn't find any obvious config entries in about:config that would relate to it either.

Does anyone have any ideas why it doesn't work in FFP and how I can get it to work?