Omenkitty, by default you can cycle forwards and backwards through Firefox tabs in numerical order with ctrl+tab and ctrl+shift+tab. Alternatively, you can use ctrl+page up and ctrl+page down. You can instead set ctrl+tab to function more like alt+tab for applications by doing the following. Go to Tools menu / Settings then search for "Ctrl+Tab cycles through tabs in recently used order" and enable it. Note, even with this setting enabled, you can still cycle forwards and backwards through tabs in numerical order with ctrl+page up and ctrl+page down.