This test highlights an error that occurs in Firefox (3.6 and 4.0) using the css properties display: -moz-box and max-width:
In Firefox the image don't respect the max-width property while Webkit does.
Moreover I would suggest to decrease the max-width property with Firebug. You will notice that the image's height change and the width remains the same.
Some references, just in case:
The CSS 3 Flexible Box Model
Flexible Box Layout Module