After install flashplugin-nonfree (aptitude install flashplugin-nonfree) we get an error saying that the file http://people.debian.org/~bartm/flas...sion.pl.gz.pgp cannot be downloaded. Ok, the problem is that the server is using https not http. So all we have to do is edit the update-flashplugin-nonfree file located at /usr/sbin:

root@cicada_3301:~# nano /usr/sbin/update-flashplugin-nonfree

And change http for https at the line:
downloadurl=http://people.debian.org/~bartm/flashplugin-nonfree/D5C0FC14/get-upstream-version.pl.gz.pgp

so the line must be:
downloadurl=https://people.debian.org/~bartm/flashplugin-nonfree/D5C0FC14/get-upstream-version.pl.gz.pgp

Sorry for my English