You must install the Freshwrapper, because Firefox doesn't support the Pepper API. There are two ways: install from PPA (recommended) or from source.
sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install freshplayerplugin
sudo apt-get install build-essential git cmake pkg-config libglib2.0-dev libasound2-dev libx11-dev libgl1-mesa-dev liburiparser-dev libcairo2-dev libpango1.0-dev libpangocairo-1.0-0 libpangoft2-1.0-0 libfreetype6-dev libgtk2.0-dev libxinerama-dev libconfig-dev libevent-dev libegl1-mesa-dev libgles2-mesa-dev ragel
git clone https://github.com/i-rinat/freshplayerplugin.git
cd freshplayerplugin
mkdir build
cd build
cmake ..
make
cp libfreshwrapper-pepperflash.so ~/.mozilla/plugins
Now you need to install the Pepperflash Plugin. There are two ways.
Download Google Chrome Stable, Beta or Unstable. That's it!
sudo ln -s /opt/google/chrome/PepperFlash /opt/google/chrome-unstable/PepperFlash
ATTENTION: The Pepperflash plugin WON'T be updated automatically! You have to update it manually!
sudo apt-get install pepperflashplugin-nonfree
sudo update-pepperflashplugin-nonfree --install
You can even use beta and dev versions of the plugin by adding --beta or --unstable.
You can skip this step and you are done! Configure the freshwrapper to enable 3d support for example. Download this file and save it to ~/.config/freshwrapper.conf.