How many tabs do you have open?
If there are a lot of tabs open then only the active tab has a close button.

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

See also:

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

• http://kb.mozillazine.org/browser.tabs.closeButtons (1 for all tabs)
• http://kb.mozillazine.org/browser.tabs.tabClipWidth

Very strange - the Tab close buttons have reappeared - perhaps it was a quirk in the System and I should have rebooted before asking the question - thanks for your response anyway - Regards

I've seen other strange things with the tab bar in Firefox 7 like new tabs appearing to the right of the new tab plus button and tabs not showing on the tab bar that requires to use the List all tabs button to access them.

Yes, me too - I've noticed new Tabs appearing to the right of the newe tab plus button - thanks again for your response. Regards