See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

I looked for updates and I have the latest graphics driver already installed.

(Important note - I have the Adobe Flash plugin set to "Ask to Activate") Three times I tried starting Firefox in safe mode and each time, the "tap here to activate plugin" window jumps to lower-right corner as soon as I scroll down >= 1 page length. When I scroll vertically back to top, the "tap here to activate plugin" window jumps back to the original (and correct) position.

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} A small dialog should appear. Click Start In Safe Mode (not Refresh). While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe websites. Are there any problems?

Then restart.

Cleared cookies as instructed above. Cleared cache as instructed above. Hardware acceleration turned off as instructed above. Problem still exists. I've included two screenshots to help.

Thanks!

http://www.foxnews.com/politics/2016/08/27/in-heartland-iowa-trump-vows-to-help-farmers-urban-minorities-alike.html

I found the issue. The web page is set up so that if you move down the page, the player stays visible. Contact the website support about this.