This issue can be caused by the Ask.com toolbar (Tools > Add-ons > Extensions)

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

• Don't make any changes on the Safe mode start window.

See:

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

Ok i have put the Firefox in Safe mode and now the "new tab" button is working. Do I have to keep it that way (safe mode) without any add on, in order the new tab button to work??? What do you advise me to do???

Definitely the ask.com toolbar. Not sure how it got installed on my pc, am guessing it "automatically" was installed when an update was performed on some software but I know for sure I did not "choose" to install it.

Anyways, simply uninstall it and you will get your tab functionality back. I hate software like this.

I'm having this problem but I do not have the Ask.com toolbar installed!

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

• Don't make any changes on the Safe mode start window.

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")

See:

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