For example, FF3.5 requires an explicit </script> tag within an XHTML document with proper DOCTYPE. The following document was validated with W3C XHTML validator …
… but it does not load the script in FF3.5.
Before anyone starts telling me that it’s not so hard to include the closing </script> tag in your source – try telling that to the database with native XML support where I’m storing snippets of the code. I had to convert the field from XML to TEXT (and lose all XML goodies I might eventually get) just to avoid the elimination of the explicit closing tag.
May I make a quick suggestion to the FF developers: maybe, just maybe, you might want to consider supporting existing web applications in parallel with adding new features that not too many people can use (because no other browser supports them yet).