If the sites are family friendly do you want to mention what they are?

Also, since you are running HTTPS Everywhere, be aware that in some cases invalid or nonexistent secure certificates could cause invisible load failures.

Sure, mainly I am having trouble with www.soccerhighlightstoday.com, but occasionally the problem affects other sites as well.

Most sites aren't affected; for instance, YouTube still works fine.

http://www.bundesliga.com/en/ has been touch-and-go.

On Soccer Highlights, because I have Flash set to "click to play", I first need to activate Flash before the black boxes show any content. There is an infobar which slides down or I can use the plugin notification icon (looks like a small Lego block) in the address bar. (Screen shot #1, #2)

Next I click to play and I get an ad. Of course. Followed by content. So it should work.

When you say you disabled all your add-ons, do you mean you tested in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions (but not plugins) and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any difference?

This did help. I had to restart in safe mode, and then reset all my features, but now everything is working fine. Thank you!