The solution to use REGEDIT in the question linked below almost fixed my trouble with PDF files on over 30 computers, with several users on each computer: https://support.mozilla.org/en-US/questions/1013642 But I needed to fix it for ALL users on a computer, without logging onto each account, on each computer. My fix was to logon as administrator and change the following system-global key instead: HKEY_LOCAL_MACHINE\Software\Classes\.pdf Name: Default Type: Reg_SZ Data: FirefoxHtm Changed the data from FirefoxHtm to AcroExch.Document.11 I use Adobe Reader XI on Win7 Pro, 64-bit machines. The problem started when I ran a downloaded file to install the new version of Firefox on the computers. Why would Firefox require me to use REGEDIT in this manner to fix the problem?