Sorry, but this most likely is due to the protected mode feature of the Flash player plugin. That feature has security benefits, but seems to have serious compatibility issues on some systems. You can disable it using the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, find "Shockwave Flash" and click the More link. Then uncheck the box for "Enable Adobe Flash protected mode" and try that for a day to see whether it helps.

I searched a little further after posting this. I went ahead and added Adblock+ and Flashblock to my browser, and it seems to be working ok for now. Some minor issues as I figure out how to get some videos I want to see to play, but over time I think I'll get the hang of it.

I may disable these two for a bit and try your suggestion, and see if that works also.

Just checked, and I'd already unchecked that the other day, and still having issues. Guess I'll stick with what's working for now.

You can check for issues caused by plugins and set plugins to "Ask to Activate" on the "Firefox menu button/Tools > Add-ons > Plugins" page.

• plugins are not affected by Firefox Safe Mode

You can check for problems with current Shockwave Flash plugin versions and try this:

• make sure you have the latest Flash version:
  http://www.adobe.com/special/products/flashplayer/fp_distribution3.html
• check for updates for your graphics drive drivers
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration
• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)
• disable hardware acceleration in the Flash plugin
• disable hardware acceleration in Firefox
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

See also:

• http://kb.mozillazine.org/Flash#Troubleshooting
• https://support.mozilla.org/kb/adobe-flash-protected-mode-firefox