Visit the following link
Select appropriate OS and download version as tar.gz
Click download button .
Open the terminal and move to your download location . In my system the default path for downloads is /home/user/Downloads
you can see your downloaded flash archive in the name of “install_flash_player_[VERSION]_[OS NAME].tar.gz
Extract the file by following command
tar -xvzf install_flash_player_11_linux.x86_64.tar.gz
now you can see 1 new file and 1 new folder extracted from the archive.
Move the file to your mozilla plugin directory
mv libflashplayer.so /usr/lib/mozilla/plugins/.
and move the folder inside /usr directory.
cp -r usr/* /usr
Thats it .. Restart firefox. To check your current version on adobe flash player plugin visit the following link.