Are you talking about this? https://addons.mozilla.org/en-US/firefox/addon/fabtabs/

if so, make sure it's installed (check under: tools >addons) if it's there and it doesn't work, leave a message for the addon developer about it on their addon page (linked above)

says will work with 4 and 6 ???

Maybe try re-installing it

You can force an update for any extension by right-clicking an extension and using Find Updates, in the Add-ons Manager tab. { Ctrl + Shft + A }