Well, this is interesting. I opened up the Storage tab of the Web Developer tools after I'd logged in, and could see the cookie from perlmonks.org -- there's a single key, 'userpass', with a hashed value. Cool.

Then I opened another tab, opened up the developer tools to the same Storage tab, and again navigated to perlmonks.org. I was prompted to log in -- but the cookie for the site was still there in Storage -> Cookies. I just compared all of the fields, and they appear to be identical.

So .. I don't know what's going on. I think I may try the same thing with Chrome on my MBP.

Update from Chrome and MBP: Chrome on OSX works fine. This may be a Firefox issue.