BBC IPlayer no longer works after Firefox update to 3.6.8.

I can confirm this as well, and also when I run from the command line I get these errors

*** NSPlugin Viewer *** WARNING: unhandled variable 18 (<unknown variable>) in NPN_GetValue()
*** NSPlugin Viewer *** WARNING: unhandled variable 18 (<unknown variable>) in NPN_GetValue()

Thank you for reporting this to Ubuntu. Please provide the results of the following command run in a terminal:

apt-cache policy flashplugin-nonfree swfdec-mozilla mozilla-plugin-gnash adobe-flashplugin flashplugin-installer

This will let us know what flash players you have installed so that we can better diagnose your issue. Thank you.

david@david-linux:~$ apt-cache policy flashplugin-nonfree swfdec-mozilla mozilla-plugin-gnash adobe-flashplugin flashplugin-installer
flashplugin-nonfree:
  Installed: (none)
  Candidate: 10.1.82.76ubuntu0.10.04.2
  Version table:
     10.1.82.76ubuntu0.10.04.2 0
        500 http://gb.archive.ubuntu.com/ubuntu/ lucid-updates/multiverse Packages
        500 http://security.ubuntu.com/ubuntu/ lucid-security/multiverse Packages
     10.0.45.2ubuntu1 0
        500 http://gb.archive.ubuntu.com/ubuntu/ lucid/multiverse Packages
swfdec-mozilla:
  Installed: (none)
  Candidate: 0.8.2-1ubuntu2
  Version table:
     0.8.2-1ubuntu2 0
        500 http://gb.archive.ubuntu.com/ubuntu/ lucid/universe Packages
mozilla-plugin-gnash:
  Installed: (none)
  Candidate: 0.8.7-0ubuntu1
  Version table:
     0.8.7-0ubuntu1 0
        500 http://gb.archive.ubuntu.com/ubuntu/ lucid/universe Packages
adobe-flashplugin:
  Installed: 10.1.82.76-1lucid1
  Candidate: 10.1.82.76-1lucid1
  Version table:
 *** 10.1.82.76-1lucid1 0
        500 http://archive.canonical.com/ubuntu/ lucid/partner Packages
        100 /var/lib/dpkg/status
flashplugin-installer:
  Installed: (none)
  Candidate: 10.1.82.76ubuntu0.10.04.2
  Version table:
     10.1.82.76ubuntu0.10.04.2 0
        500 http://gb.archive.ubuntu.com/ubuntu/ lucid-updates/multiverse Packages
        500 http://security.ubuntu.com/ubuntu/ lucid-security/multiverse Packages
        100 /var/lib/dpkg/status
     10.0.45.2ubuntu1 0
        500 http://gb.archive.ubuntu.com/ubuntu/ lucid/multiverse Packages
david@david-linux:~$

Sometimes Firefox profiles can become corrupt and/or extensions can
conflict. Could you please try this with a new profile and see if the
problem still exists? You do not need to delete your old profile to
test. If a new profile helps, you might want to try to disable all your
extensions and re-enable them one by one until you find the problematic
one.

You can test a new profile by starting profile manager with the following command, but make sure all Firefox windows are closed first:
firefox -ProfileManager

To disable extensions, Tools > Add-ons > Find the extension >
Highlight and click 'Disable' > Restart Firefox

Thanks Drew212, using a new profile does indeed fix the fault. Going back to the original profile, I can disable ALL extensions and it will still not work. Is it possible that version 3.6.8 is not compatible with existing profiles saved under earlier versions? This kind of thing does happen on Firefox - I can remember a while ago when a new version would not use saved passwords from the previous version...

Try moving compatability.ini out of the profile, it should cause your plugins to reregister. Hopefully that fixes the problem.

No, that has no effect on the problem - compatibility.ini is automatically recreated but the profile still exhibits the bug.

Just installed new Firefox version 3.6.10 - problem solved!