Firefox does not actually bundle any version of Flash (unlike Chrome) - it is always a separate package.
Yes!! Chrome has it built in and but FF doesn’t.
Firefox does not come with any flash plugin out of the box on Ubuntu MATE.
I’m sorry for the confusion but I tried to check on a live usb thumb but instead booted from a SD card (which has UM installed on it, updates and etc.).
It's too easy install the latest Adobe Pepper Flash plugin for Firefox, Chromium or Vivaldi :
· Ubuntu/Linux Mint/Debian-64bit:
Open a Terminal 
and write these ones...
sudo apt-get update
sudo apt purge --remove adobe-flashplugin pepperflashplugin-nonfree flashplugin-installer
sudo apt-get install pepperflashplugin-nonfree
.......if this one doesn't work write this: sudo apt-get install adobe-flashplugin
sudo apt-get install browser-plugin-freshplayer-pepperflash
If you installed adobe-flashplugin(step 4b) write this: sudo rm -f /usr/lib/adobe-flashplugin/libflashplayer.so
Close and open your browser again.
September/2017