I just upgraded from Firefox 3 to 5 and find that my blog is shown without a stylesheet at all. It still works fine in Firefox 3, and latest Chrome and Safari and I assume other browsers too. And the rest of my site still works fine in Firefox 5 too; it's only the blog part that's broken.
Update: It appears it was the stylesheet that was not recognized. I had my stylesheet inside an .asp file to be able to execute server-side code in it. That has always worked just fine but it appears...