[CentOS] What the heck is a "text/html decoder"

My phone service provider offers a way to check my voicemail via a webpage.  However, when I go to that webpage with Firefox and try to play back a saved message, a window pops up that says this:

"The following plugin is required: text/html decoder"

What is it looking for and how can I install it?  I already  have these totem rpms:

totem-nautilus-2.28.6-2.el6.x86_64
totem-pl-parser-2.28.3-1.el6.x86_64
totem-mozplugin-2.28.6-2.el6.x86_64
totem-2.28.6-2.el6.x86_64

and the following gstreamer rpms:

gstreamer-ffmpeg-0.10.11-2.el6.x86_64
gstreamer-0.10.29-1.el6.x86_64
gstreamer-devel-0.10.29-1.el6.x86_64
gstreamer-plugins-good-0.10.23-1.el6.x86_64
gstreamer-plugins-bad-free-0.10.19-2.el6.x86_64
gstreamer-plugins-bad-free-extras-0.10.19-2.el6.x86_64
phonon-backend-gstreamer-4.6.2-26.el6_4.x86_64
gstreamer-tools-0.10.29-1.el6.x86_64
gstreamer-plugins-base-0.10.29-2.el6.x86_64
PackageKit-gstreamer-plugin-0.5.8-21.el6.x86_64
gstreamer-plugins-ugly-0.10.18-4.el6.x86_64


-- 
MELVILLE THEATRE ~ Real D 3D Digital Cinema ~ www.melvilletheatre.com