Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

To Enable SafeMode

• You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
• Once you get the pop-up, just select "'Start in Safe Mode"

If it works in Safe Mode and in normal mode with all extensions (Tools > Add-ons > Extensions) disabled then try to find which extension is causing it by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: "Firefox > Quit Firefox"; Linux: "Firefox/File > Quit")

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thank you. I have done as you suggested, twice now. The first time, just exiting & restarting with one add-on enabled at a time and sending a new email to me at yahoo.ca. No problems encountered at all. Then I repeated that and rebooted the computer as well each time. Again no problem seen, other than the fact my Yahoo Mail still requires 2 entries of the password. I will see what happens. Maybe the problem is fixed, maybe not?????? If it shows up again I will contact you, OK?

Maybe also do a malware check with some malware scanning programs on the Windows computer.
Please scan with all programs because each program detects different malware.
All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

I have the latest versions of both Norton 360 and Iobit Malware Fighter, and both are run frequently. They always find "something" and both report my computer is 'clean'. I appreciate the thought, thanks.

I "Reset" Firefox, and that seems to have cured the Survey/Fatal Error message. I still have to sign in to Yahoo Mail twice every time. This is something recent and is only on this computer, on my other one, I only have to sign in to Yahoo Mail once as I always had to previously on this one. I am closing this query. Thanks for your help.

This can be caused by corrupted cookies or cookies that are blocked (check the permissions on the about:permissions page).

If clearing cookies doesn't work then it is possible that the cookies.sqlite file that stores the cookies is corrupted.

Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookie files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted.

• http://kb.mozillazine.org/Cookies
• https://support.mozilla.org/kb/Deleting+cookies

I have received several suggestions re this problem but yours seemed to produce the best results. However, the problem has returned and I have decided to uninstall Firefox, completely, and then do a new installation. Are there any things I should do other than just 'uninstalling' the program, to ensure that the new installation will have no holdovers from before? Any suggestions will be appreciated.

If you uninstall Firefox and choose to remove personal data then Firefox will remove all profile folders.
You may want to backup your personal data if you choose to do this.

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile

You can also try to create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem.

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