Portable Flash player does not work in last updates, before it worked well.
It does not show show in plug ins
I tried copy flash files in "CommonFiles\Plugins", and also into "\FirefoxPortable\Data\Plugins" and no result.
Please test it, and tell if it works on your system:
Flash may require an install on the local machine on newer versions of Firefox. I have not tested in a while. I know it works in Firefox Portable when Flash is installed locally at present. Make sure you're using 64-bit Flash on a 64-bit machine and 32-bit on a 32-bit one. Firefox Portable includes both and automatically switches.
Sometimes, the impossible can become possible, if you're awesome!