Another feature of Thunderbird that I suspect has just been broken in version 60.x is the "Fetch headers only" option.
For as long as I can remember, this has been my setting of choice, but now its broke!
I think I accepted an upgrade to my Lubuntu system just before departing for a weeks holiday, centred around Mold in North Wales ...a whole week of digital detox, it was great!
Now I'm back I find that when I check for new email, the headers are downloaded as expected, but when I click on the Download rest of the message link I get nothing ...nowt.
I've been using Thunderbird to collect my mail using POP3 and initially only download the headers. If I don't like the look of the email from its header, I just delete it. If it looks kosher, I go ahead and download the message body and take a look at it.
After a couple of hours of playing around with settings, and a fruitless search of the net, I decided it was probably broken, but maybe no one else has noticed.
My workaround is to got to Account Settings > Server Settings and select Automatically download new messages and de-select Fetch headers only.
Note that this enables you to read all new emails (i.e. those downloaded after you make these changes) but you still won't be able to download any mail where you had previously only downloaded headers. For those messages you need to access them via your web-mail interface.