I use Thunderbird mainly on ubuntu 18.10 and run currently version 60.5.1. When I try look for an email, I usually type the search word in the search box at the top right and then sort the results by date. This works fine on another PC on which I run Thunderbird under Win10, with similar (probably the same) settings for email accounts. Most of them use IMAP, a few use POP3. However, when I do the same on the linux machine, nothing happens when I click on the "sort by: relevance date" button. Whether I click on "relevance" or "date", it's always sorted by relevance.