I am posting this for other users’ reference. Obviously NPAPI plugins are used less and less thankfully and really are no longer a requirement especially flash. But some websites still require flash.
For example if you want flash-plugin to run on the nightly FLATPAK version of Firefox please see the following steps:
1: Download the flash tar ball from https://get.adobe.com/flashplayer/
2: Extract flash_player_npapi_linux.x86_64.tar.gz to a location of your choice inside your home directory
3: Inside the folder flash_player_npapi_linux you will find the file libflashplayer.so. Copy this file to the following hidden location in your home directory:
4: Open Firefox and enter about:addons in the address bar. Under the section Plugins you will now see Shockwave Flash listed and you can enable it
Remember you can install the rpm package for flash from https://get.adobe.com/flashplayer/ and flash will work on the rpm version of Firefox but won’t work on the FLATPAK version as this version is sandboxed