The initial release of desktop zooming will only allow zooming in relative to the 100% zoom level, not zooming out past that, which matches the behaviour of other desktop browsers.

The ability to zoom out past 100% could be useful for pages with wide content, and we could consider enabling it. We have some support for it behind the apz.allow_zooming_out pref, but there are issues like bug 1647177 to iron out before it can be shipped.