The plugin version in question is "Square", flashplayer10_2_p3_64bit_linux_111710.tar.gz

Please read this thread:

http://support.mozilla.com/en-US/questions/767695

Ok. Read.

That's an Ubuntu support page. I'm running Debian. We do not have packages for Firefox so "apt-get install flashplug-nonfree" just does nothing. There is no "Firefox" in the Debian distribution, just "iceweasel". I am running "real" Firefox 4, which I installed by hand.

Another thing I've tried: replacing the 64-bit version of the plugin with the 32-bit version, including adding /usr/lib32/mozilla/plugins/ (/usr/lib -> /usr/lib64) already.

Still need help.

If you have a 32 bit Firefox version then you need to install a 32 bit Flash plugin. A 64 bit Flash plugin won't work with a 32 bit Firefox version.

You can create a plugins directory in the Firefox installation directory and place the plugin or a symlink to that plugin file in that directory if other locations of the plugin aren't working.

So what I was saying was that neither the 32 bit version nor the 64 bit versions of the plugin are recognized by FF4.

The installation directory was /opt/firefox. This is the first place I put the libflashplayer file and it did not work.

Other help?

I did add the "contrib" distro to my Debian sources so that flashplayer-nonfree is now installed, and indeed iceweasel now can run Flash. But Firefox 4 cannot. The iceweasel plugin is in /usr/lib/iceweasel/plugins/libflashplayer.so, but as I already said, /usr/lib/mozilla/plugins/libflashplayer.so is STILL not recognized by FF4.

In Firefox 4 you can also place plugins in a plugins directory in the Firefox Profile Folder.
So you can try to create a plugins folder there and see if that makes Firefox see the plugin.

Help > Troubleshooting Information > Profile Directory: Open Containing Folder