I have Ubuntu 6.0.6 and I can only see videos in firefox if they can be opened with real player -but no sound comes out. Otherwise I can not see them at all.
I have tried to install the plug in using easy ubuntu but there is a bug in the current version that means it will not install - it gives an error about broken packages.

Your sound problem might be that some components are muted. You can check your sound system on your PC using Check if you see any MM. If so, you can unmute it by pressing the m key.

For the video output, you might want to check which type of software you have to see videos. I use "xine" to view roughly any type of video file.

Cheers

Sadly that did not solve the problem.

Do you have sound with anything else?

Yes. I have mananged to solve the sound problem (I will post later as I do not have the details).
However now if I watch videos it uses multimedia and has sound but no video.

I fixed it by going here http://ubuntuguide.org/wiki/Ubuntu_dapper
and down loading everything that could help me. Sorry I know that is not very helpfu. But after I changed my sources.lst these are all the program I down loaded :-

258 apt-get install vlc vlc-plugin-* mozilla-plugin-vlc
259 apt-get install mozilla-mplayer
260 sudo apt-get install totem-gstreamer-firefox-plugin
263 apt-get install alsa-oss
264 vi /etc/firefox/firefoxrc
265 apt-get install avahi-daemon
266 apt-get install avahi-daemon && apt-get install avahi-daemon
268 apt-get install alsa-oss
269 vi /etc/firefox/firefoxrc
270 apt-get install avahi-daemon
271 apt-get install avahi-daemon && apt-get install avahi-daemon
272 apt-get install flashplugin-nonfree
273 apt-get update-flashplugin
274 update-flashplugin
276 sudo update-flashplugin
277 apt-get install gstreamer0.10-ffmpeg gstreamer0.10-gl gstreamer0.10-plugins-base gstreamer0.10-plugins-good gstreamer0.10-plugins-bad gstreamer0.10-plugins-bad-multiverse gstreamer0.10-plugins-ugly gstreamer0.10-plugins-ugly-multiverse w32codecs

This was all as root and I don't know why I applied sometwice. If you know how to put all those updates on one command then that solve all your problems if change your sources as in that link
(You do have to make some manual changes as well