Minecraft – How to reopen the new minecraft launcher

I was finally able to determine with the help of some of my forum friends that Minecraft's source code has been updated so as to not allow it from being played in an applet to prevent some inconspicuous bug whereby people who hadn't payed for the game were able to play it in an applet. This fortunately reinvigorated my efforts to determine what was wrong with playing the regular launcher. I was eventually told by the IRC chat for Minecraft that it was my ESET NOD32 antivirus which was causing the problem, and that I was forced to uninstall it in order to play Minecraft and install a different antivirus. I have elaborated on this in the comment I made below, but my problem is unfortunately not yet resolved. Here is where I posted the bug on the official Mojira bug posting site. https://mojang.atlassian.net/browse/MCL-887 However as the original problem has been solved I'll simply leave this as the answer.

Yes! And very easily, too. Profiles allow you to tell Minecraft where to store all its files by changing the Game Directory option in the profile. Just add something memorable onto the Game Directory (such as 1.7-snapshots) and that profile will never see your existing worlds.

How this works is, setting this to a different folder than the default .minecraft folder makes the game keep an almost entirely separate Minecraft install in the specified folder. (The only thing it doesn't keep separate is the game code itself – this is still drawn from the .minecraft/versions/ folder, which as you've found works just fine and doesn't compromise firewalling different installs.)

I use this myself to maintain multiple private modpacks separately from the main Minecraft version, and they co-exist perfectly and peacefully within the official launcher. It means that there is absolutely no danger of accidentally opening a modded world while running vanilla Minecraft (and thereby destroying the save!) or vice versa. I make it easy on myself by simply adding modpacks/[name-of-modpack] onto the end of the default Game Directory, which neatly stores all my modded Minecraft stuff in a "modpacks" folder within my .minecraft folder.