HOWTO Get a load of hard-disk space back

Boing Boing: “A handy tip — if you use Thunderbird to get your email, don’t forget
to occasionally run File -> Compact Folders. I did so yesterday and
reclaimed nearly 20GB of hard drive space! Comparing my mail folder to
my backup, I discovered that every single email that I’d “deleted” for
over a year (by putting it in the Trash and then emptying it) was still
lurking on my disk.”