Errr, well, if those bookmarks exist in IE, then it will happily import them for you, regardless of what they are. Did you check to see if they existed in IE when you imported them?
Well, I never saw them, but I will need to investigate. I just downloaded Firefox on my computer at work, and there were no offending bookmarks. Probably something specific to my laptop at home, and not endemic to Firefox. I'll check it when I get home.