Librem 13v3; PureOS with updates through 6/2.
It appears that Thunderbird has not been updated in a while. The current version is 52.4.0, which appears to have been released in 10/17. From 'apt list':
thunderbird/green,now 1:52.4.0-1 amd64 [installed]
The recent 52.8 release has some fixes to address the EFail issue. Personally, I would be fine waiting for version 60 slated for this month, but I do not know how long that will take to go through the pipe once it is available.
Thanks for your attention.