We've been cleaning up the basement and I put some things for sale on Craigslist. Every time I tried to log in to the site with Firefox, I'd get a 404 error. I have Internet Explorer installed for situations like these, but it's not ideal.
Tonight, I discovered the culprit: Craigslist really wants the referer header to be set. I had switched it off in Firefox a while ago (with network.http.sendRefererHeader = 0); switching it back on fixed things.