Gack, what a mess my blog feed has been in. It's still not 100% okay, but at least Firefox now parses it without complaint. Maybe someone with more XML-fu than me will read this and tell me how to fix the rest...
I've always gone to a fair bit of trouble to ensure that my blog's HTML is syntactically correct, at least in terms of HTML, and everything is spelt correctly. To achieve this I use a fairly involved script to perform a bunch of sanity checks when I make a new post. Unfortunately, I've been neglecting the state of my blog's feed, and as I've recently discovered, it's been in quite a bad way for quite some time. Given that more people probably read by blog via the feed than the HTML, I've probably been focussing my attention disproportionately.
It seems that whilst the native HTML I'm writing may be valid 4.01 Transitional, the XML that Blosxom generates from it isn't valid XML, mainly in terms of tag closure. So I guess this is going to mean that I need to go for a stricter and/or newer standard. I might need to move to XHTML or something.
I've also found that a lot of the HTML generated by the various blog meme generators is utter crap.