Thanks for sending that information. I took a look, and found some interesting results.
-- all page data loads (which I suspected, given that you could see all data in Safari or Opera). If you load one of the problem pages in Firefox and view the page source, you'll see that the full content is there; it's just being cut of display-wise.
-- this suggests to me that it's a theme compatibility issue, although I'm not sure why Vanilla or ClassicNavy would be doing the same (I checked on my own install, they act normally under Firefox). Have you made any direct edits to any of the default stylesheets?
-- debugging user-generated themes is beyond our free support scope, but I can give you a few suggestions:
1. Using Firebug
(which you may want to try -- it's great for debugging css issues), it looks like the display problem is with div id "main". Check to see what you have in your custom theme for its display options, and try messing around.
2. Try fully deleting the theme to see if ClassicNavy and/or Vanilla are fixed.
3. When trying any of this, remember to hard-refresh your browser, to make sure the new stylesheet loads. (On Mac computers, you can do this by holding down the Shift key while clicking the Refresh button.)
Let us know if you have any other questions!