Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)

• You can open the Firefox 4.0+ SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
  

Don't select anything right now, just use "Continue in SafeMode."

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

When you figure out what is causing that, please let us know. It might help other user's who have that problem.

Thanks for your reply. Still working on it too. With all plugins disabled, on one site I now see a commercial clip that shows before the requested clip , but video does not play (using Admin mode, just thr opposite, don't see commercial, but see vid)

This I believe is due to plugin AdBloc (which like very much, thank you) , but I don't see this as actual problem.

Question: I know it is recommended to NOT be logged as Admin for security, but how dangerous is it to run FF as Admin to "trusted" (?) sites ?

For now this seems my only option

Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.org/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Question: I know it is recommended to NOT be logged as Admin for security, but how dangerous is it to run FF as Admin to "trusted" (?) sites ? (have UAC enabled)

I have created "new user", but get same results as previously noted. I ran FF logged in as Admin on site http://animal.discovery.com/videos/puppy-bowl-viii-highlights/ with AdBlock disabled- got ad clip immediately followed by video; safe to assume AdBlock is not causing problem.

Disabling ShockWave Flash plugin prevents all videos from playing, so must be player involved Any thoughts on setup or preferences for this plugin?

Being logged on as Administrator merely avoids having to enter the Administrator password when needed.
You get a UAC request (dimmed window) to ask for permission when required.
If you would be a normal user then you would have to enter the Administrator password as well to confirm an action, so it is easier to confirm an action by accident.

Same question, different approach

Why doesn't Adobe Flash Player show videos in one profile, but works okay in another?

My apologies; I found I have same problem using IE 9 & Google Chrome; (1 profile under normal user - other is under administrator Vista SP2)

Apparently means NOT a FF related problem

Even so, any ideas??

yep, sure enough opening Firefox in safe mode ensures that Flash does not work at all... - Sorry I am not clear on what the point of that exercise was?

as with dshand, Flash works fine when I use my Windows account but not when I log in using my wife's account. I have tried re-installing Adobe Flash, and disabling and re enabling the plug ins. I have also tried creating a new Firefox profile when logged in as my wife - this does Not play flash videos either.