Firefox version 19.0 comes with an embedded PDF viewer. However, this viewer failed to properly display PDF files that have Greek text and have been prepared with XeLaTeX. I have contacted the PDF.js developers who were very helpful and suggested to install the development version: http://mozilla.github.com/pdf.js/extensions/firefox/pdf.js.xpi to solver the problem. Indeed, this version solved the problem and now I can read my PDF files with no problem. Thank you people for this great piece of Open Source software!