For Lucid, solution to play Quicktime videos (including URL previously referred to is:
-Delete the plugins registry file. That should be in your ~/.mozilla/firefox/<profile (like xxxxxxxx.default)> folder, where ~/.mozilla is generally /home/<yourname>/.mozilla. Delete the file pluginreg.dat, which firefox will recreate when it starts.
-Using Synaptic Package Manager, ensure that the multiverse repository is enabled.
-Remove the totem mozilla plugin named totem-mozilla.
-Install gecko-mediaplayer, which automatically installs all of the mplayer goodness as well.
PS I have vlc installed (for playing of DVDs as it's the only app that always works for that) but I do not have the vlc-mozilla-plugin package installed.