Now that I re-read my reply, it could simply be that the application software that is generating the wiki (some sort of PHP content management system I'd guess) is case sensitive, and for all I know it could be running on Windows Server 2008. So mentioning Linux at all is a huge red-herring. Pretend you never saw it.
But since you say both links work for you... no Firefox on Vista still treats the links differently (one works, the other doesn't). Or at least on my machine it does. Weird. When you say it works, you mean it has all the content too right? Because a page comes up on both links, but one has no content (the second one I listed)... that's because a wiki doesn't have physical pages, it just takes the last bit and uses it as a key to find content in a database. When that key isn't correct, the page renders, but without any content. Just like when you type nonsense at the end of a wikipedia URL (for instance [Invalid or Expired Link Removed]) it doesn't fail to load the page, it just says "I haven't got anything on this yet, would you like to make an entry?". That's what's happening here, but it's more insidious because at a quick glance FEARful_FAQ and Fearful_FAQ look identical. And in systems that are case-insensitive, they ARE identical, because case doesn't matter then.
Ok, way more info on this subject than anybody wants to know I suppose. After I wrote wiki over and over, it finally dawned on me I could do something to improve the situation, so I added a link on the "bad" URL to the correct one. Duh...