sudo pacman -S flashplugin
Then if you launch firefox, you will find out that there’s no sound when playing flash in it. Launch it from the terminal, you will find out the reason.
Failed to open VDPAU backend libvdpau_va_gl.so: cannot open shared object file: No such file or directory
So you need to install
sudo pacman -S libvdpau-va-gl
Launch again, everything is fine now.