I just found out that to view Mathematical Markup Language (MathML) properly in Firefox, you need to download the fonts.
As for IE 6, external plugins are needed to view MathML properly.
And if you want to check if your browser can support MathML, go to this test page.
For list of supported browsers, see this.
For not-so-apparent reasons, IE always seems to have problems following W3C's recommendations. I wonder if IE 7 will support XHTML 2.0 and MathML (unlikely, Microsoft will most likely leave it to external plugins to "demonstrate" the "flexibility" of IE).
I just looked at the IE blog.
If I ever get to install IE 7... the first thing I will do is to find a way to fix that MSN Search to Google Search.