how "portable" is "portable"?

I was just wondering if anyone had any experience moving an existing Portable Firefox installation folder from one USB device to another. I tried to copy an image of one USB drive to another, larger one. All of my other portable apps are completely happy on their new home drive (as I expected they would be), but Firefox is not.

When launched from the original thumb drive, everything is still OK. But on the new drive the migrated installation is "broken", even after repeated attempts and methodologies to copy it. The program seems to launch/run just fine, which is nice. But, none of the extensions are seen and the toolbars/buttons/options aren't set up like they are on the original drive.

Now, I would expect you'd run into this sort of "wonkiness" if you were masochistic enough to try moving a program folder between, say, two different Windows XP boxes. You know, on the new machine you'd be missing registry entries, maybe customization files squirrelled away in the My Documents folder, and other little hidden-but-needed bits that would get left behind on the old machine and not transferred to the new one.

But, I would have expected Portable Firefox to be bullet-proof to this sort of manipulation, as I was under the impression that all needed files "lived" only on the USB device. How does the Portable Firefox app (and associated customization files) even "know" it has been moved? The two drives, apart from their size differnce, are identical. I'd hoped the transfer would have been "transparent", but Firefox is wise to my shenanigans, and is not cooperating. I'm at a loss to figure it out, but hoped maybe some guru here would have some insight as to why it's misbehaving.

Thx,

S-M