I was using gnome-tweak-tool and momentarily bound Ctrl+Tab to one of the window-switching navigations. After clearing the rebinding, it didn't go back to doing what it used to do: switching tabs within a window like Firefox's.
I'd like to get this behavior back. Does anyone know how? I'm in Ubuntu-Gnome 14.04 LTS; not sure what other information would be helpful.
settings>keyboard>shortcuts
and look carefully at every section for your key combination. If you find any shortcut mapped to Ctrl+Tab, change it. My shortcuts do not contain any Ctrl+Tab setting and in Firefox it does switch tabs as expected.