For those brave enough to expose their system complete with security vulnerabilities to the vagaries of the Internet, you can find the original 64-bit flash plug-in, which was withdrawn by Adobe. You can also download it at Softpedia.
Caution: Use it at your own risk. (Fools rush in where angels fear to tread).
The Debian forums page explains the process or there's a Debian Flash Wiki with blow-by-blow account. You can also check out the Debian Flash Bug Report.
Essentially you have two options:
1. Use chroot, install and run a 32-bit browser
2. Use a wrapper with 32-bit plug-in and run a 64-bit browser
The second is an easier solution, though it can be buggy and there are some minor issues.
If you decide to go down the chroot road, there's a Debian 32-bit chroot HowTo. As you can see, there's a truck load more work involved than the nspluginwrapper option.
"You pays your money and you takes your choice."
Here's a walk-through for the nspluginwrapper method:
1. Add debian-mulitmedia.org repository to /etc/apt/sources.list
2. Download gpg key for aptitude or apt-get
3. Install the key ring
4. Update aptitude or apt-get
5. Install the 32-bit libs needed for 32-bit plug-in running on a 64-bit system
6. Install flashplayer-mozilla
7. Restart your browser
You must be root. Answer "yes" to the prompts. You can copy each line of text and dump it into an XTerm or other terminal. Save ya having to type it out.
Here's the code:
1. echo "deb http://www.debian-multimedia.org squeeze main non-free" >> /etc/apt/sources.list
2. wget http://www.debian-multimedia.org/pool/main/d/debian-multimedia-keyring/debian-multimedia-keyring_2008.10.16_all.deb
3. dpkg -i debian-multimedia-keyring_2008.10.16_all.deb
4. aptitude update
5. aptitude install ia32-libs ia32-libs-libnss3 ia32-libs-libcurl3 libcurl3 nspluginwrapper
6. aptitude install flashplayer-mozilla
7. Restart Firefox/Iceweasel.
I used this method on my existing 64-bit Firefox/Iceweasel and it works.
As a foot note, I should add I normally run with flashblock to kill flash-ads. Only time I run flash player is when I visit YouTube, and that's rare.