Step 1
go to http://get.adobe.com/flashplayer/
select .tar.gz for other linux
and then click on download to download it

Step 2
open the terminal and go to the Downloads folder
extract the source by command
#tar -zxvf install_flash_player_some-version_linux_i386.tar.gz
a file libflashplayer.so will be extracted from source
copy that file to the mozilla plugins folder by following command
#sudo cp libflashplayer.so /usr/lib/mozilla/plugins

done.