Mozilla just released another final version of it’s Thunderbird version, 52.6.0 comes with minor changes and corrections. At this time I wrote the article, Mozilla still links to the older 52.5.2 version.
The releasenotes can be found here together with the system requirements here.
The latest 58 beta can be found here - releasenotes here.
It’s a pure bugfix version as you can see from the releasenotes but I’m thankfully that these guys working on the best eMail client ever! Mozilla v58.x is almost done and people getting the new “Photon” UI very soon as default new look.