We will update RHEL 7.x to NSS 3.28, which is incompatible with Mozilla 50 and older versions, depending on which features are enabled in NSS RHEL.
For RHEL 7.4.0, we will update nss-softokn, so it's certain that this issue affects all Mozilla applications, which are kept at versions at Mozilla 50 or older.
On 7.3.z and older, we'll most like keep the older nss-softokn version, so this might be a non-issue
I recommend that you pick up a small patch in Thunderbird, and other applications such as xulrunner (if we're using any such applications in RHEL), to prevent potentially issues.
The issue is not so much 3.28 as the X25519 that this version adds, and since the implementation of x25519 is in softokn, that is incompatibility with nss-softokn 3.28 package
This does not apply anymore.