Before you implement
Cache-Control: immutable, you better have a good URL structure with versioning or other methods of cache invalidation.
When Firefox 49 shipped it contained the Cache-Control: Immutable feature to allow websites to hint which HTTP resources would never change. At almost the same time, Facebook began deploying the server side of this change widely. They use a URI versioning development model which works very well with immutable. This has made a significant impact on the performance of Facebook reloads with Firefox. It looks like other content providers will adopt it as well.