Thunderbird 52.0 has been released on 2017-04-04. The latest version is 52.0.1 released on 2017-04-14. https://www.mozilla.org/en-US/thunderbird/52.0/releasenotes/ https://www.mozilla.org/en-US/thunderbird/52.0.1/releasenotes/
52.0.1 was released to fix an conflict between it and McAffee anti-virus scanning on Windows. There's no advantage to rolling this release for linux. 52.1.0 is in the works and will be released very soon, however.
Ian, thank you for the update. Here is the latest development version for the impatient who wants to look at the ebuilds: https://gitweb.gentoo.org/proj/mozilla.git
commit 044af98f97ba5a2621bffef857d5f2a8aa8aa878 Author: Ian Stakenvicius <axs@gentoo.org> Date: Wed May 3 10:18:28 2017 -0400 mail-client/thunderbird: bump to 52.1.0 This version defaults to the cairo-gtk3 toolkit by default.