CSS: wp-calendar caption tag in Webkit and Mozilla (and Opera/Presto)

I am making some small maintenance changes to my CSS and PHP templates, to improve some of the formatting, and I’ve noticed the following oddity. Any help on how to fix it would be great.

I’ve got the following in my CSS:

#wp-calendar caption {
	font: bold 1.3em Verdana, Arial, Sans-Serif;
	text-align: center;
	color:#6A5ACD;
	}

When this renders in Safari/Chrome or Opera, the month caption on top of the calendar centers nicely above the calendar in the center. However, on Firefox/Camino, it aligns to the right.

What causes this? And how can I get FF and other Mozilla browsers to render it properly?