I had mentioned this issue in a previous blog post, and I think I have identified the root cause. If you use Firefox to browse Facebook, at some point, content will begin to fail to load. This is because of malformed URL often generated by AJAX history management libraries when they do URL replacement or add tags to the URL to help them keep track of history (essentially manage what happens when you click the Back, Forward, or Refresh buttons on the browser). You can do two things about it - either reopen the link in a new tab (so there's no history), or edit the URL as I'll show you.
(1) Fire up the fox and log into Facebook. Your URL should look something like this:
(2) Click on the "Messages" link. It should open your messaging area, defaulting into your inbox. The URL should look like this:
(3) Click on the "Home" link. It should bring you to your Facebook home page. The URL should look as in the image below. Notice how the URL looks a bit weird - and that's the beginning of the problem. The more you browse around, the more URL chunks are appended at the end of the current URL.
(4) Click on the "Profile" link. The URL looks like that in the image below. This will fail to load content most times, as I described in that blog post.
If you want that URL to work, remove the part between the domain and usually before the last #. So, http://www.new.facebook.com/home.php#/profile.php?id=73002299 becomes http://www.new.facebook.com/#/profile.php?id=73002299, for example, and it will work.
This is only one of many ways Firefox users will run into this issue. Any combination of browsing history will most likely come to the same conclusion, and it's not pretty. Facebook totally needs to fix this bug, or they risk frustration from Firefox users that with think their site sucks when their favorite browser is used.