When I'm using Firefox, my Foxit Reader has been randomly popping the sign 'can't read the PDF file' when I haven't been trying to read any PDF files, especially when I'm clicking the "Apply" button in Indeed.com and when I'm on Amazon.com.
Because of this problem I tend to avoid FF and use IE, but IE has another problem: it doesn't change the home default website as I tell it to.
Try Sumatra PDF. Light weight and quick. You don't even have to install it if you get the portable version. Just unzip portable version and place anywhere. Change .pdf files to open with Sumatra.exe and your golden.