I need help with the same question. I already reset NVRAM and SMC.
I used Cmd + Shift +, which did reveal some files in [hard drive]/Applications, but files in [username]/Applications and [username]/Library/Internet Plug-Ins still remain hidden.
I set this in Terminal:
defaults write com.apple.Finder AppleShowAllFiles true
Nothing new happened that I can tell.
Still need to find these files, which are probably in [username]/Applications and [username]/Library/Internet Plug-Ins
Login Item: /Applications/Skitch 2.7.8/Skitch.app/Contents/Library/LoginItems/J8RPQ294UB.com.skitch.SkitchHelp
er.app
Login Item: /Applications/Snagit.app/Contents/Library/LoginItems/SnagitLaunchAtLogin.app
Plugin: /Library/Internet Plug-Ins/CouponPrinter-FireFox_v2.plugin
Plugin: /Library/Internet Plug-Ins/o1dbrowserplugin.plugin
Plugin: /Library/Internet Plug-Ins/nplastpass.plugin
Plugin: /Library/Internet Plug-Ins/googletalkbrowserplugin.plugin
Plugin: /Library/Internet Plug-Ins/CouponPrinterPluginMac1.0.0.7.plugin