Wayland is a modern replacement, but it is not a drop-in. That is because it has a completely different architecture and approach.
I cannot ELI5 the differences between the two, because they are somewhat vast.
If you are on a platform that supports Wayland, it is undoubtedly a smoother, prettier and overall better experience. This falls apart due to the lack of direct support from certain apps, desktop environments, etc. The migration has taken some time - but alas this is an open source effort in a massive/fragmented environment so it is obviously a very hard challenge. Like herding 100,000 cats.
Unfortunately not everything will just work. A stopgap is something called xwayland which allows x apps to communicate with wayland - although it is not perfect.
Firefox will work with Wayland. It will work with X. It will work with XWayland. The default though is X or XWayland. You normally need to explicitly tell Firefox to boot up using Wayland. Firefox is now considering making it enabled by default, which would be great for those of us who run on Wayland all the time. It would of course still work on X.