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The first thing that would come to mind is invalid code of some sort. Is your install of FF 3.5.2 having trouble with other feeds as well, or is it just this particular feed?
Why not take the validator's advice and address the MIME type issue?
Another option might be to generate an Atom feed instead of RSS with the same tool. When I hand code I use RSS by habit, but if I have a tool that will generate a feed for me I'll often go with Atom. It has a bit more functionality available compared to RSS.
it says I should change the mime type because feeds should not be served with the "text/plain" media type. I haven't done that yet (not knowing exactly what to change it to) but I don't think that will solve the problem
It will solve the problem. You need to use the mime type
for RSS 2.0 feeds - the details of how to achieve this depend on your server environment (the problem is a result of a server misconfiguration).