Try:

• http://kb.mozillazine.org/Corrupt_localstore.rdf

You can also check the value of the browser.tabs.closeButtons pref on the about:config page.

• http://kb.mozillazine.org/browser.tabs.closeButtons

• http://kb.mozillazine.org/Resetting_preferences

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration 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

Also are you using any Tab related extension?

yes I know safemode was mentioned above which would disable such Tab extension if used.

Thanks for the responses.

I started Firefox in safe mode and sure enough the 'x's reappeared.

I then closed and restarted normally intending to try disabling add ons one by one to find which caused the problem.

When I restarted normally the 'x's were there on each tab.

Have attached a screen shot of my active add ons.

Looks like the safe mode restart cleared something.

I disabled all my addons (NoScript, AdBlock, XMarks and YouTube Comment Snob), and the "x" re-appeared. Then I enabled the add0ns one by one, and the "x" stayed. I have all four addons enabled, and my "x" on the tabs is still there.