See if moving the "Open a new tab" button in right position solve the problem of missing tab. Go to View > Toolbars > Customize and with left click hold and drag the "Open a new tab" button(the + button) in the right position, then click Done, exit firefox and restart it.

see for more info : Toolbar customization

thank you

Please mark "Solved" the answer that really solve the problem, to help others with a similar problem.

Thank you for replying.

Nope. There is no "Open a new tab" in customization since the button is already there. 2nd, I did a restore default, but that did not help. The tab is still missing.

Drag the button you allready have (the + button) from left to the right, but to do that you must open the Customize menu.

thanks again

I've seen such issues myself.
In such a case new tabs that I open appear at the right side of the new tab button.
This looks like a bug to me.

You can usually correct this by closing the (last) tab with Ctrl+W and restoring that tab via Ctrl+Shift+T

Not working.

The + button is already on the right in customize menu.

Cor-el solution works, but it has to be done every time the browser is re-open. Plus, the + button is in between the first and the rest of the tabs. So, I would say is a bug. I will log it in as a bug in bug tracker.

Only functional workaround that I have found is to enable the "group tabs" button. Then click it and click on the image in the upper left hand corner. Yucky, but at least functional.