Those funny little boxes

So, you've seen those "little squares" in IE (especially when visiting foriegn language sites), right? You've also seen those little boxes with a 2x2 array of characters in Firefox. What are they? Just missing fonts/characters? But wait, you don't have any special characters that visitors to your site would need additional fonts to view. What gives?

Those boxes indicate that the browser was unable to find a character, and so looked for the character in a different font set. Sometimes, it finds a match. When it doesn't, you get the "little boxes" (more detailed explaination here)

Those numbers within the box (in firefox) indicate the unicode for the missing character.

Here's a good site for searching unicode:
http://www.fileformat.info/info/unicode/char/search.htm

So, per that search,  (0 0 0 backwards E in a box in firefox) = control key