MultiCookie is a browser add-on that allows you to have multiple saves.
A recent update introduced the prestige system, and following this update many users wanted to have multiple saves (one after a reset and one without a reset). With how the game is working, there are two main ways for multiple saves: A. Use another browser. B. Open two tabs in the same browsers. The problem with the first solution is that, well, you are using your current browser for a reason, it's pretty silly to use two browsers at the same time. The problem with the second solution is that in case your computer accidentally shuts down, you will only recover one save (this is beacuse the two saves are using the same location for their data, and they both replace each other when they save). MultiCookie provides a way to have multiple saves without these problems.
More information Edit
For more information (and download links) you can visit MultiCookie's page on GitHub.