If you have a blog powered by WordPress and you’re using a theme based on “Default” or “Kubrick” you need to change to another theme.
The issue is that these themes do not play nicely with many of the HTML5 parsers in forthcoming browsers.
The bad news is that WordPress shipped with default themes based on Kubrick up until the recent 3.0 WordPress release, so there are hundreds of thousands of sites using these themes.
In many cases these sites will not display properly on Firefox 4.0, Chrome 9, Opera 11.5, or IE9. They do work in Firefox 3.6 and other earlier browsers.
If you’re running a WordPress powered blog, right now would be a good time to download the Firefox 4 beta and check your site.