Thunderbird still has a critical bug open that literally deletes your email. I have been bitten by it once and lost hundreds of email permanently. Not fun. I am subscribed to the ticket and every month or so someone posts an outraged "me too". https://bugzilla.mozilla.org/show_bug.cgi?id=462156
https://bugzilla.mozilla.org/show_bug.cgi?id=462156#c141 : "This bug has lasted this long because there is no reproducible test case, so thus no way for an engineer to diagnose the problem and test a patch." I sympathize, but I'm not sure what you expect the team to do. The bug seems highly intermittent even when it does occur and there doesn't seem to be anything in common between the systems that are affected.
i mean, as a user this would suggest that the product is outright not fit for purpose.
i would expect the team to include a clear and obvious warning on the homepage (preferably near the download link) that emails are subject to deletion due to this bug.