Try to open a new window in case the current window has a problem.

You can still try Safe Mode because sometimes extensions hide themselves and do not show in the extensions manager.

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

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

It is also possible that security software is causing this problem

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that helps.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

If this didn't help then try to use the Profile Manager to create a new profile.

Create a new profile as a test to check if your current profile is causing the problem.

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 files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Note that your System Details List shows multiple Flash plugins.

1. Shockwave Flash 15.0 r0
2. Shockwave Flash 17.0 r0
3. Shockwave Flash 19.0 r0

You can find the installation path of all plugins on the about:plugins page.

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

You can check the Flash player installation folder for multiple Flash player plugins and remove older version(s) of the plugin (NPSWF32) and possibly (re)install the latest Flash player.

• (32 bit Windows) C:\Windows\System32\Macromed\Flash\
• (64 bit Windows) C:\Windows\SysWOW64\Macromed\Flash\

Thank you for your time to give the suggestions above. Here is the status of me going down your list of things to do: Problem persisted when opening a new window, and also when I started FF in Safe Mode. However, I could open new tabs in Windows10 Safe Mode. This is good news (I think?), but where does this leave me? Thanks again for your help, Steve V.

What security software do you have?

It is possible that your security software is blocking the new tabs. You can temporarily disable your security software to see if that is the culprit and if that is the case check the setting for Firefox in your security software

Security software seems to be what's built into Windows10, namely Defender. I switched it (Defender) off and then launched Firefox, however I still could not open a new tab - so problem remains.

But doesn't exist in Safe Mode. I don't know what to do next. It would be good if Windows, when launching, would give a list of what other software is being installed - drivers, apps, dlls, etc.

Can you make any further suggestions regarding things to try?

Thank you for your time, Steve V.

To cor-el, regarding suggestions posted above: Per your suggestion I did create another Profile, but still the problem persisted - could not open a Tab. Thus far it works only in Safe Mode. Regards, Steve V.

Do you see any extensions installed when you start Firefox in Safe Mode?

• Firefox menu button or Tools > Add-ons > Extensions

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

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:
  http://windows.microsoft.com/en-us/windows/using-defender
• 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

Dear cor-el,

I'm now happy! Thank you very much.

New Tab now works. This is due to me running the first malware program you listed above, Malwarebytes. I'm thankful you suggested it since I wouldn't have thought this would be the problem (because the problem occurred immediately after a MS Windows7 update.

Anyway, thanks again for sharing your time and expertise.,

Steve V.

You're welcome Steve.