I recently reinstalled Debian Lenny after experimenting with Slackware for a bit. Everything works just how I remember it should, except for plugins for Iceweasel. One of the first things I did, of course, was install the package flashplugin-nonfree. It seemed to have installed correctly and it showed up in /usr/lib/mozilla/plugins, but nothing was shown in aboutlugins when I checked from within Iceweasel. I managed to install the flash player from the Adobe website just fine, though, and it works. I then tried to install the mozilla-mplayer plugin. The same thing happened as with flashplugin-nonfree. It's weird. But maybe I'm just missing something really easy.

First question are you running 32bit or 64bit?

What does the command dpkg -l | grep -i flash show?

Does ls -l /usr/lib/mozilla/plugins/*.so
list flash?

What does update-alternatives --display libflashplayer.so show?

If you are running 64 bit the following should fix it, as root do: If running 32bit just do the second command
# dpkg-reconfigure nspluginwrapper
# update-alternatives --auto flash-mozilla.so

You should see something like:
#update-alternatives --config flash-mozilla.so
There are 3 choices for the alternative flash-mozilla.so (providing /usr/lib/mozilla/plugins/flash-mozilla.so).

Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/lib/flashplugin-nonfree/libflashplayer.so 50 auto mode
1 /usr/lib/flashplugin-nonfree/libflashplayer.so 50 manual mode
2 /usr/lib/gnash/libgnashplugin.so 10 manual mode
3 /usr/lib/swfdec-mozilla/libswfdecmozilla.so 10 manual mode

Just make sure flashplugin-nonfree is being used and not swfdec or gnash if you have it installed.

I'm running 32-bit. The output of the grep command is
The first plugin listed is the one I installed from the .deb on the Adobe website, the one that actually works and shows up in aboutlugins.

The ls command does indeed list flash, both of the versions.

Update-alternatives says that there are no alternatives for libflashplayer.so, so I can't configure it.

Graduation is a stressful and busy time, but I finally got around to looking at my problem some more. I did more googling and found this thread:
http://www.linuxquestions.org/questi...plugin-692082/
Post #13 was strangely helpful and now flash works after I copied libflashplayer.so to the other location. But the mplayerplug-in still does not work. All the .so files are in /usr/lib/iceweasel/plugins and /usr/lib/mozilla/plugins and the .xpt files are in the components directories. I can't figure out what the problem is. Luckily, mplayer is less important than flash.

Get rid of the adobe-flashplugin package, it is built for Ubuntu and not debian. Search the forums here for how to remove it.

Well, I actually did remove the adobe-flashplugin package and forgot to mention that I had. Just to be clear, flash now works exactly how it should. The problem now is mplayer.

All of the iceweasel plugins should just work.

aptitude install mplayer mozilla-mplayer
make sure you do not also have a different plugin like vlc-plugin, or totem-mozilla trying to play the same media

I know the Iceweasel plugins should just work. Which is why I'm confused that they aren't. I did install mozilla-mplayer using aptitude, exactly how I'm supposed to, but it just doesn't show up in aboutlugins. flashplugin-nonfree had the same exact problem until I copied libflashplayer.so into /usr/lib/iceweasel/plugins, but after I did that, it works just fine. So I'm confused. And I just tried installing a different plugin to see if maybe it was just an mplayer problem. It's not. mozilla-plugin-vlc doesn't show up in aboutlugins either after I install it.