I have a WordPress multi-site, and my problem is that Firefox can't open any of the PDFs that I have there. I get a dark-gray screen and it says "This PDF document might not be displayed correctly", but I am able to download it. In .htaccess I have this rewrite rule for files:
`RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]
The thing is that if I write the actual path to the file it opens it in preview just fine. Also, Chrome and IE don't have this problem.