Probably after upgrade an extension created the problem, try step by step : Troubleshooting extensions and themes to find the problem.

If there is not problem with extensions try to modify browser.tabs.closeButtons in about:config .

type in the address bar about:config

then press the button i'll be careful i promise

then type in the bar browser.tabs.closeButtons

then right-click on it, select modify, put 1 (probably you have 2)

click OK, exit firefox and restart-it.

see for more info : browser.tabs.closeButtons

thank you

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Aha! Before starting out with these instructions, I closed Firefox and restarted. When I did this, it updated once again, to 10.0.2. Now my X is back. That was easy! Thanks for the suggestions.

SOLVED

I have the same problem. I know it's not an extension as I have the same setup at work and don't have a problem there. It's not 10.0.1 as it remained after 10.0.2. And the browser.tabs.closeButtons is already set to 1. The interesting thing is that *sometimes* I have the X on the tabs and other times I don't.