iMac 2009 with El Capitan 10.11.6 has two user accounts I will call A and B. The 3 browsers are Safari 10, Firefox 56 and Vivaldi 12. Yesterday when I logged into iMac user A and clicked to open Firefox it crashed immediately without opening. This repeated several times so I ran Disk Utility but that did not help. I ran Bitdefender, yes it has found malware and a virus in the past, and it found nothing. I uninstalled Firefox, restarted, ran DU again then downloaded and installed a fresh Firefox. The same thing happened. Safari functioned as expected. I switched to user B and all was normal. I downloaded Vivaldi and it crashed at start in user A but worked normally in user B. Next I compared user accounts and saw no notable difference in the system data. Next I downloaded Etrecheck, ran it and in Configuration Files a note in red said "/etc/sudoers, File size 2411 but expected 2299" which is beyond me and I could not find that file from looking through the system. What am I overlooking or not seeing? I looked into reinstalling El Capitan and hit a brick wall because El Capitan can only be installed through Terminal and my nerve damaged hands along with lack of experience and training in Terminal negate that approach.