Right click the jar file and click "open with" then do Shift + cmd + g, type "/System/Library/CoreServices/Jar Launcher.app" and make sure that it's set to "Always open with" and then java
Whenever I try to open up the forge jar(for 1.12.2) to install it, all it does is expand the file. This did not happen when I installed 1.11. Is this a problem you are able to help with or is it something with my computer?