The redirect error has been covered before, and I thought So10 had fixed it but I just had it happen again on bbpress.org/forums/ .
I was not logged in, and at the bottom of the thread there was a message "you must log in to post" or something similar (the link was at the end of the thread for sure.) So, I clicked that and got a page with two login forms, essentially: the normal login one, then the one that appears when you make a mistake on your password. I used the top one (form class="login", with grey background) to enter my info, then tabbed over to the submit button and hit enter. It submitted the form, then I got the Firefox error "This page is not redirecting properly" (see screenshot)
Oddly, even though that Firefox error is displayed, I was logged in. When I use the browser back button, I see that I am logged in. Also, oddly, if I use the lower form (the form has no class, but has tr class="error" for the first row) in the page to enter my login information, and submit that, the redirect error does not happen and I am logged in normally.