Installing the Flash plugin manually

Go to the Flash Player download page at Adobe.com.
When prompted, save the file (i.e.
install_flash_player_"version"_linux."processor".tar.gz).
At the top of the Firefox window, click on the File menu and select Quit
Open a Terminal window (in Gnome, click on the Applications menu,
select Accessories, and then select Terminal.)

In the Terminal window, change to the directory where you saved
the file you downloaded (e.g. cd /home/user/Downloads).

Extract libflashplayer.so from the file you downloaded with the command

tar -zxvf install_flash_player_"version"_linux."processor".tar.gz.

As the super user, copy the extracted file, libflashplayer.so, to
your Firefox installation directory's plugins sub-directory. For
example, if Firefox is installed in /usr/lib/mozilla, use the command

sudo cp libflashplayer.so /usr/lib/mozilla/plugins

and then enter your super user password when prompted.