W^X ("Write XOR Execute"; spoken as W xor X) is the name of a security feature present in the OpenBSD operating system. It is a memory protection policy whereby every page in a process' address space is either writable or executable, but not both simultaneously. from wikipedia.
The new Nightly Mozilla Firefox come with enabled the security feature W^X .
Also will be it available to other versions of its web browser once they are upgraded to version 46.
The implementation of W^X makes all Just in Time ( named JIT) code pages working with the browser.
This will needs to write to pages, a function needs to be called to explicitly make the page writable.
Also that will remove the execute flag at the same time.
The good think is the permissions for memory pages which allows the compiler to patch code without performance overhead.