Goto http://getfirefox.com/ and download the latest one.

 tar -jxvf firefox-*.tar.bz2
 sudo mv firefox /opt
 sudo dpkg --add-architecture i386
 sudo apt-get update

 sudo apt-get install libc6:i386 libstdc++6:i386 libxrender1:i386 libasound2:i386 libdbus-glib-1-2:i386 libxext6:i386 libgtk2.0-0:i386 libxt6:i386 libcurl3:i386 libnss3:i386 libnspr4:i386 libssl1.0.0:i386

After that,

  /opt/firefox/firefox
  should work.

Next get the 32 bit tar.gz file for adobe flash, unpack and copy it to /opt/firefox/browser/plugins/

 tar -zxvf install_flash_player_11_linux.i386.tar.gz
 mkdir -p /opt/firefox/browser/plugins

 cp libflashplayer.so /opt/firefox/browser/plugins

and you should be good to go!