Need to invoke the "logout" function on the web browser (not the server).

Unfortunately, modern browsers are making this harder to find/use. I can't see any easy way to do it in Chrome.

From Firefox, click "tools", "clear private data", select "everything" in time range to clear, unclick all options except "Active Logins" (guessing here), click "clear now". Not tested. Will log out all HTTP authenticated sessions, not just the one you want.

I seem to recall, once upon a time, this use to be easier. Perhaps do I google search in case there is a better way I have missed. It does seem though that HTTP authentication has been forgotten in favour of session based authentication and as such isn't getting the features it deserves.