I had set my preferences to use MathML, but I've just realised it fails to display square root of fractions correctly. With MathML, $$\sqrt{\frac{n-2}{1-r^2}}$$ looks almost the same as $$\frac{ \sqrt{n-2}}{1-r^2}$$ In particular, the square-root encloses only the numerator, which changes the interpretation of the formula
I'm using Firefox 3.6.10 under Windows XP Pro Service Pack 3. I selected MathML by right-clicking any displayed formula to display a context menu and selecting Settings => Math Renderer => MathML.
Apologies if this is a MathML problem rather than a StackExchange problem. I don't know where to raise MathML bugs though.
EDIT: Snip of screenshot as requested: