The task not the program

It's not just the task - it's the fact that the Thunderbird interface is stateful.  

Take the message filtering box - a nice feature for power users, but confusing to novices, who
will type something in here and then later on wonder why all their messages "have been
deleted" or "have vanished".  This is truly a panic-inducing prospect for someone who is a
novice at Thunderbird and not very confident with computers.  Making it really obvious how to
"show all my messages again" would help solve this - or have a "progressive disclosure" model
where this feature is not enabled initially but can be turned on with an "I am a power user"
toggle set by the user.

I'm not a usability expert so there may be much better solutions to this, but this just shows
that there can't have been much usability testing of Thunderbird.