In Firefox on Windows 7, closing tabs by clicking on them with the middle mouse button doesn't work. You only get the Alt+Tab 'window' to switch between programs.
To fix this:
- Go to Control Panel
- Select 'Hardware and Sound'
- Select 'Mouse' under 'Devices and Printers'
- In the drop-down box for 'Wheel button' change it from 'Flip' to 'Middle-click'
- Press OK
This should now enabling closing tabs with clicking the middle mouse button.