Does Memory have to get swapped in to be freed?

["Martin S. Weber" <Ephaeton%gmx.net@localhost>]

Hoi,
   Just an observation after killing an ages old firefox with
a good count of plugins having run. I look at my xosview and
watch disk activity for quite a period, with rising memory
usage. Then at a sudden the disk activity stops, all the
memory (obviously used by firefox) is freed and memory
usage drops quite, while finally the ffox window closes.
   What is kind of strange is the time of disk activity
which kind of goes around the system and ties it up while
swapping in. Just to free the pages swapped out? Isn't it
possible to free those pages just on the swap, without
reading them in? Or was I witnessing something absolutely
different?
   (4.99.60/i386/2008Q1 ffox)

-Martin