User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.87 Safari/537.36

Steps to reproduce:

Not sure exactly what triggered the issue. The only notable thing I did was trying to search for a mail that contained a specific phrase and and I tried doing so in Sent and then All Mail folders.

Messages were being downloaded via ProtonMail Bridge
https://protonmail.com/bridge/thunderbird

The about window showed version 68.1.x (don't remember the x). Since then it has been updates to 68.4.2. Confusingly there are higher versions listed in the bug version dropdown field.

Actual results:

Thunderbird was downloading messages. Eventually I noticed that counter on the bottom bar has passed the count of total messages.

Expected results:

The counter of downloaded messages should not have increased over total count of messages.

I wonder if the first number could be the UID of the message, while the second number shows how many messages you currently have in that folder/inbox.

Dunno, but from user perspective it certainly looked wrong.