From what I would guess, it might be rather that these pages do not conform to what Firefox expects from a page to be suitable to be shown in reader mode. Large pieces of text with a few pictures in them or something like that. If you check back in a few hours, I might have posted more information on this.
It's got to be something like that, unfortunately I just don't know the exact syntax. I've got some other javascript bookmarklets in firefox, but looking now at their links, they are a mess of %-encoding, and I'm not sure if that's important or not (perhaps they got mangled through an old import)
FWIW, here's a bookmark to get rid of static bars (the kind of thing annoying websites have stuck at the top of the page and won't go away when you scroll):