Download Flash Player .xpi version from the official Macromedia flash site.
download using IE or google chrome so Firefox would not install it.
Rename the resulting flashplayer-win.xpi file as flashplayer-win.zip and extract it using your favorite compression software like WinZip or 7Zip etc.
Copy the resulting flashplayer.xpt and NPSWF32.dll files into the folder plugins folder.
(see below note)
Restart your Firefox Portable/ Portable Chrome browser to enjoy the flash plugin in it. You can check if the flash is installed by going typing about:plugins in the URL bar
GOOGLE CHROME PORTABLE's plugin folder is GoogleChromePortable\App\Chrome-bin\plugins - you will have to create it if not found.Then copy flashplayer.xpt and NPSWF32.dll
FIREFOX PORTABLE plugin folder is \Data\plugins\