Firefox is a web browser distributed with Xubuntu, which is extendible using plugins.
To install Adobe Flash Player plugin for Firefox, use the command:
sudo apt-get install flashplugin-installer
Get the last .tar.gz version of Adobe Flash Player from https://get.adobe.com/flashplayer/, then open a terminal in the extracted folder and use the commands:
sudo cp libflashplayer.so /usr/lib/mozilla/plugins/
sudo cp -r usr/* /usr