See Tab preferences and settings for help.

I unchecked "OPEN NEW WINDOW IN A NEW TAB INSTEAD" . It made no difference.

Have you tried restarting the browser?

Yes. I even uninstalled Firefox and re-installed it , to no avail.

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

Please report back to see if this helped you!

Uninstalling and reinstalling is not recommended - it would've been better if a reset was done. Now it is recommended that you do a clean reinstall as instructed above.

I am afraid it didn't work. Maybe I should change browser because I find this problem very annoying. Thanks.

Does this affect particular sites, such as Google? I mention Google because it has a setting to launch each result in a new window, which Firefox can divert to a new tab.

Do you have any tab-related extensions such as Tax Mix Plus? You can check on the following screen and either try disabling them or adjusting their settings using the Options button:

orange Firefox button (or Tools button) > Add-ons > Extensions category

There was an extension "Open Bookmark in New Tab" that I disabled and then deleted . The problem was solved only after I clicked on re-start after the deletion. Thanks a mill because I was beginning to despair both from the annoyance and from the idea that I have to abandon Firefox.

To answer your question : all sites were affected. I guess this is now obvious but thought I'd mention it to make sure.