Can you try to start Firefox in Safe Mode to see if Firefox works properly with no error? You can start Firefox in Safe Mode below:

• Windows/Mac: Go to Help > Restart with add-ons disabled
• Linux: Run firefox -safe-mode in the Terminal/Konsole

If Firefox opens up fine with no problems, it's probably one of your extensions that's causing the issue. You can re-enable your add-ons one by one until you find the one that causes the issue upon being re-enabled.

I found that after Firefox v29, a LOT of my settings and
add-ons were changed / reset. Try this; Start Firefox in Safe Mode
While you are in safe mode;
Firefox Options > Advanced > General.
Look for and turn off Use Hardware Acceleration.
Then check ALL of your settings. Browser and add-ons. Then restart.

Please elaborate on this comment: "Then check ALL of your settings. Browser and add-ons." What do you mean by "check" settings, browser, add-ons??? How? What am I looking for? Thank you.

The problem was not resolved when I restarted in safe mode.

Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back to us!

Open the Firefox options; Press the <Alt Key> to bring up the tool bar. Then Tools > Options Go to each page in turn. Make sure that all the options are the way you want them. Also; Options > Advanced > General. Look for and turn off Use Hardware Acceleration.

After you are done, open and check your add-ons. Some of them have options. Press the <Alt Key> to bring up the tool bar. Then Tools > Add-ons > Extensions.

Thanks!!!!!

I downloaded/ran Malwarebytes. If found and quarantined the following:

rogue.antivirSolutionPro rogue.antivirusSuite.Gen

...And I no longer have the issue.

Thanks again for your help!