Be careful when playing with clientWidth and masking SVG elements.
Each element "hidden" in <defs> will have a zero clientWidth, clientTop etc. in Firefox.
<svg xmlns="http://www.w3.org/2000/svg" version="1.1"
width="100%" height="100%" xlink="http://www.w3.org/1999/xlink">
<ellipse
id="IdoHaveClientWidth"
rx="10%" ry="20%" cx="50%" cy="55%">
</ellipse>
<defs>
<mask>
<ellipse
id="IhaveNoClientWidth"
rx="10%" ry="20%" cx="50%" cy="55%">
</ellipse>
</mask>
</defs>
</svg>
To get the real dimensions, you'll have to clone the element and insert it as a direct child of the SVG tag. This applies to the current Firefox version - 22.0.