"Add in a new Tab" and "Add in a new Window" are completely unresponsive since upgrading from 3.1.7 to 4.0.1; how do I get them to work again?

Start Firefox in Use Troubleshoot Mode in Firefox to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Thanks for the suggestion. I re-started Firefox in safe mode and it worked correctly. I then re-started in normal mode and it continued to work correctly. Re-booted, etc., and it continues to work; no need to disable any plug-ins. I guess something was just 'stuck'. Thanks again.