I have windows 8 and after downloading jar files (specifically mods for minecraft) i receive the file in my downloads folder or my desktop (tried using both). Firefox, Chrome and IE give me the same problem. I've disabled my norton firewall/antivirus protection when downloading to try and help. Someone suggested going into about: config in Firefox and changing Browser.download.manager.scanwhendone to false however in the about: config page i searched for it and it wasn't there. When i go to Firefox it tells me i'm using the latest version. Other files (such as downloading and installing Firefox) work just fine and it seems to only be jar files. I have tried opening them from the miniature popup in the browser and from their download file locations. Every time i would run/doubleclick or open with java, absolutely nothing happens. Just yesterday (6/21/2014) i was able to run the same file type in any normal way. I do not have windows 8.1 however when i went to reset my computer i did "restart and update". When i go to my java folder i have jre7.
Can't run downloaded jar files?
Find Java on the Win 8 equivalent of the start menu. Use the option to configure Java.
Click the Security tag and set the level lower than it is.
Click the Advanced tab.
Enable the console
Choose an Application Installation setting which will work for you - you might need to play around with them.
Enable:
allow user to grant permissions to signed content
Show sandbox warning banner
Don't prompt for client certificate selection when no certificates or only one is available
Warn if site certificate does not match hostname
Mixed code:
You probably want "Enable - show warning if needed"
Go over the Advanced security settings and set those according to your desired risk/protection level.
You may find further information here:
http://www.wikihow.com/Run-a-.Jar-Java-File
Also, make sure that jar files are open by javaw.exe in the file associations list.