Installing flashplugin 11 on Ubuntu 11.10 (firefox)

First of all, download the tarball for flashplugin 11 from here.

Untar it, in whichever directory you want. It will create a subdirectory called "usr" and a single file called libflashplayer.so

You must copy the contents of this usr directory inside the real /usr:

sudo cp -r ./usr/* /usr

And finally, copy the single file to the firefox plugins directory:

sudo cp ./libflashplayer.so /usr/lib/firefox-addons/plugins/

Don't forget to restart firefox!