any typos in XHTML (when served as XML) will result in the yellow-and-red screen of death in Firefox.

Or, say, using && in your javascript. Yes, that does it. I have the 30 minutes of wasted time a week ago to prove it.

Here's the "solution", just so you can see how elegant it is. You have to escape it like this:

  <script type="text/javascript">
  /* <![CDATA[ */
  if (var1 && var2) blah blah;
  /* ]]> */
  </script> 

Or just use AND I suppose, forgot if it's got some weird precedence difference or something.