One of the annoying issues I was facing with mozilla thunderbird email client was that when I opt to have emails grouped into threads (conversations), the threads are sorted according to the Date and time of the earliest/oldest email in the thread. So if someone replies to a thread a month after the oldest email in that thread, the thread will have an unread message in it, but I won't notice it because it's not among the topmost few threads in my inbox.
Figured out the fix now, to have threads sorted by the date and time of the newest email in it.
View > Sort By > Threaded.
Wonder why thunderbird doesn't have this as the default setting.
Anyways, now when someone replies to an old thread, that entire thread gets brought to the top in my inbox, making it impossible that I miss a mail.
PS: I know this sounds a silly post; but if you use thunderbird, you'll know how annoying this is. In fact, google shows there are people who quit thunderbird for this singular reason.