I've been using (and updating) flash player successfully for a while now by copying just the NPSWF32.dll file to my FirefoxPortable\Data\plugins directory.
From what I've read on forums, though, you're also supposed to copy the flashplayer.xpt file. But mine works fine without it. So what does the .xpt file do exactly? Just curious. Thanks.
Edit: Could it be that the .xpt file is necessary, but FFP is reading it from the Windows\system32\Macromed\Flash directory (while reading the updated .dll from the portable directory)?