I happened to be checking the plug-ins installed in Firefox on my laptop
and noticed something that may be related to you inability to view some
videos in Firefox and IE.

Here are the steps to check:

Open Firefox.
Click on the "Firefox" tab in the upper left corner of your screen.
Then click on "add-ons"
Then click on "Plugins"

Scroll through the installed plugins to find any reference to "Flash
Player or "Shockwave" and verify that it specifically says "Shockwave
Flash" and not "Adobe Flash" player or something else without "Shock".

The reason to check this is because I just discovered that Adobe has two
different download sites for "Flash", call it two different things and
they have two different current version numbers.

If you go he http://get.adobe.com/shockwave/

it is called "Adobe Shockwave Player" Version 12.0.7.148

But, if you go he http://get.adobe.com/flashplayer/

it is called "Adobe Flash Player" Version 12.0.0.43


I thought it was weird that they would have two names and current
version numbers for the same thing. So, I thought maybe they are not
the same thing. Well, they are not.

I found this on an Adobe FAQ:

"What's the difference between the Flash and Shockwave Players?

Flash and Shockwave Players are both free Web Players from Adobe.
Together, they bring you the best rich media content on the Internet.
Each has a distinct purpose. Flash Player delivers fast loading
front-end Web applications, high-impact Web site user interaction,
interactive online advertising, and short to medium form animation.
Shockwave Player displays destination Web content such as interactive
multimedia product demos and training, e-merchandising applications, and
rich-media multiuser games. When you download Shockwave Player, it
automatically includes Adobe Flash Player."

So,

Maybe the problem you are having viewing some videos is because you only
have Adobe Flash player installed minus the "Shockwave" component.

According to the answer provided in the FAQ, if you download the
Shockwave player from http://get.adobe.com/shockwave/
you will get both components ... Flash *and* Shockwave.

If you installed Flash from the other Adobe download site, you would
only get the Flash component, minus Shockwave. Betcha it makes a
difference!


The only other thing I noticed while looking at all this stuff is that
both Adobe download sites have a note on the right hand side just above
the "Install Now" button that says "Your antivirus software must allow
you to install software."

You have mentioned that you are using a Windows provided antivirus
program. I'd make sure you temporarily turn it off when downloading and
installing Adobe. Windows is notorious for being overzealous in their
"protection" from other software applications especially if it's not
provided by Microsoft. If your Firefox plugins says Shockwave is
installed, you should be ok.