It's probably because Firefox does not support many presentation attributes as well as proper font antialiasing for the text and tspan SVG tags. It looks like they did create their own text layout engine with support for flowing around arbitrary shapes.

I'm curious how well this works, since emulating native text capabilities (just think RTL vs LTR) requires an insane amount of Javascript and the performance is pretty terrible for texts longer than a few hundred words. If they got this right, it must be a masterpiece of Javascript code. Google recently introduced their own text layout capabilities in Google Docs (also SVG based) and it's pretty buggy beyond simple text. Just to highlight the complexity of the issue, they used to render text into SVG on the server until recently.

It's one of the reasons we had to use foreignObject with pure HTML inside for Moqups instead, although it prevents us from supporting IE at all, including IE 11. We can at least target Webkit/Blink & Gecko this way.

[1] https://developer.mozilla.org/en/docs/SVG_in_Firefox [2] http://www.w3.org/TR/2013/WD-css-shapes-1-20130620/