Browsers usually don't support this, see How do I log out?
Since browsers first started
implementing basic authentication,
website administrators have wanted to
know how to let the user log out.
Since the browser caches the username
and password with the authentication
realm, as described earlier in this
tutorial, this is not a function of
the server configuration, but is a
question of getting the browser to
forget the credential information, so
that the next time the resource is
requested, the username and password
must be supplied again. There are
numerous situations in which this is
desirable, such as when using a
browser in a public location, and not
wishing to leave the browser logged
in, so that the next person can get
into your bank account.
However, although this is perhaps the
most frequently asked question about
basic authentication, thus far none of
the major browser manufacturers have
seen this as being a desirable feature
to put into their products.
Consequently, the answer to this
question is, you can't. Sorry.
There are browser extensions that allow you to clear the HTTP authentication for a site.
For Firefox the WebDeveloper extension (which is one of my favourtie extensions anyway) offers this feature.
The menu for this is Miscellaneous/Clear Private Data/HTTP Authentication.
