Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

See also:

• http://kb.mozillazine.org/browser.link.open_newwindow
• 1: current tab; 2:new window; 3:new tab;

For links opened via JavaScript you can look at this pref:

• http://kb.mozillazine.org/browser.link.open_newwindow.restriction

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

rzn6jw said

I have 'Open new windows in new tab' selected. However, when opening a new window, Firefox opens another instance of Firefox for the selection, not a new tab.

What are you clicking, exactly, or what menu choice are you using?

Ran Firefox in safe mode and the issue seems to have cleared. Now I have to figure out what happened because I have the default theme anyway and only two 'required' add-ons. Can't say which one it is right now because I'm in safe mode but I'll restore and try disabling them.

Thanks.