A note to those on 52.9.1
thunderbird 60.0 is tunderbird for quantom, its a new upgrade but you have to get it, if you have 52.91 and are happy with it, ie don't have any issues, use the link I sent to go to it in releases and get that.
However, please note that your not likely to get any more updates for it.
Its also worth mentioning that the 52x version is the last mozilla developed version it looks like while mozilla hosts it its a redone engine using the new quantom engine.
Unlike firefox bar the addons dialog, its actually quite accessible and is probably what I will use going forward.
However its worth noting that if you really don't like it you can go to 52.91 and stay there for ever more, just know there is no security for you, on the other end, your addon or addons may or may not work in the latest version.
The only thing I changed was messages to open in the same window else it remembered my settings
It also appears that it keeps the same interface themes.
I have not as yet noticed any slowdowns though.