Optimize CSS & JS breaks IE & FireFox (All Themes)
One possibility is that one or more CSS files have a Unicode BOM at the beginning.  The CSS optimizer that Drupal uses (at least the one in 6) doesn't work properly with these.  IIRC, the first rule gets mangled if it is the very first line in the file.

Switching the encoding with Notepad++ is the easiest way to fix this in you are on a PC.