Margin top in inline element

This is not Firefox-only, and defined in the CSS 2.1 Specification:

8.3 Margin properties: 'margin-top', 'margin-right', 'margin-bottom', 'margin-left', and 'margin'

Margin properties specify the width of the margin area of a box. The 'margin' shorthand property sets the margin for all four sides while the other margin properties only set their respective side. These properties apply to all elements, but vertical margins will not have any effect on non-replaced inline elements.

^{(Emphasis at the end is mine; margin-top is a vertical margin)}