[fix bug 948057] (B2G Browser) Zooming in on modal window causes graphical issues

Firefox OS performs a layout/reflow on `position:fixed` elements when the user pinch/zooms. This affects `Mozilla.Modal`, resulting in layout issues and odd glitches.

This PR switches to using `position: absolute` for small viewports, which does not exhibit the same behavior and performs more predictably.

I've tested this change on a GeeksPhone Peak device.