How do I reset the zoom level in Firefox?

Select View » Zoom » Reset from the menu.

Alternatively press Cmd + 0 on Mac OS X, or Ctrl + 0 on Windows and Linux.

Go to View > Zoom > reset
alternatively you can press ctrl+0

Go to "View" in the menu, then "Zoom" and then "Normal". Or just press 'Ctrl + 0' (Zero)