Firefox has a feature named tab groups. Since you can only see one group at a time, you might have a tab in a different group. Press Ctrl+Shift+e to open and close the tab group screen to take a look. To combine tabs into one group, you can drag them between the different rectangles on that screen.

Is that it?

If so, what is the hidden tab about?

There are other things that need your attention.

Your System Details List shows multiple Flash plugins.

• Shockwave Flash 15.0 r0
• Shockwave Flash 11.5 r502

You can find the installation path of all plugins on the about:plugins page.

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

You can check the Flash player installation folder for multiple Flash player plugins and remove older version(s) of the plugin (NPSWF32) and possibly (re)install the latest Flash player.

• (32 bit Windows) C:\Windows\System32\Macromed\Flash\
• (64 bit Windows) C:\Windows\SysWOW64\Macromed\Flash\

You can try to open a new window and use that window instead (i.e. close the currently used window).