Normally, when maintenance releases of Firefox and Thunderbird come out, the only changes improve compatibility or fix bugs – it’s rare that a new feature slips in there. However, Thunderbird 184.108.40.206 changes the behaviour of the ‘Date’ sorting field to reflect the date the email was received, not sent, which was bug 166254 in Bugzilla (and is almost 5 years old).
Normally, if you sort your emails by date (like I do), the date used is the date that the email was sent. This is fine in most cases, but falls down when the sending computer has the wrong date, the email somehow gets delayed or the date is left off the email together; the latter case means that the email gets tagged as being from 1st January 1970. Apple do this with the Developer Connection emails – most of your new mail will be at the top, except theirs which get pushed to the very bottom, which is annoying.
Thunderbird 220.127.116.11 fixes this, so email uses the date it was received. This means that, when sorted by date order, your emails will all be sorted by the date you received them, no matter what time they claim to have been sent.
I suppose you could say it fixes a bug, but as I understand it, it’s also possible to turn off the new behaviour, so that makes it a feature in my mind. And it’s one that I’ve been looking forward to.