How to clear memory?

[joe at actionline.com]

Yesterday, I tried to post this question to plug,
but it was bounced back as undeliverable.  Wonder why?

Anyway, here it is again:

My systems have begun to run very slowly.

Is there some way to clear memory without shutting down or logging out and
back in?

Actually, even when I log out and back in, they are still slower than what
has been normal.

Especially Firefox has become much slower than what has been normal in the
past.

Also, when I tried to check plug threads today, I couldn't get the plug
website to open, though other websites seem to be working fine (albeit
slower than normal).