Thunderbird "my Thunderbird inbox is non-empty" test consistently fails

It seems that since March 3, this step of “Scenario: Thunderbird can download the inbox with POP3” fails every time it runs, both on our shared Jenkins and on my local one:

    Then my Thunderbird inbox is non-empty                                            # features/step_definitions/thunderbird.rb:219
      <false> is not true. (Test::Unit::AssertionFailedError)
      features/thunderbird.feature:49:in `Then my Thunderbird inbox is non-empty'

Some of the failures were caused by the mailbox being indeed empty (no idea why) but the test still fails while I’ve ensured there’s a message there (confirmed by the test video). I suspect the Dogtail code that checks this is buggy.

Feature Branch: bugfix-16422-thunderbird-60.5.1

Related issues

• Blocks #13241 (closed)
• Blocked by #16422 (closed)

Original created by @intrigeri on 16555 (Redmine)