I sometimes get that message when going to “forums.opensuse.org”. It also happens with konqueror, and probably other browsers (for “forums.opensuse.org”). I have occasionally seen it at other sites, though that is rare.
As far as I know, it is more a site issue.
When you login to opensuse forums, you are redirected to a “microfocus” site to login and add a cookies. Then you are redirected back to the forums to add a “opensuse.org” cookie. I assume that the two cookies are sometimes out of sync, and that causes the redirection loop.
When that happens here, there are several option:
Close “firefox” and restart it;
delete all “opensuse” cookies and all “microfocus” cookies;
open a private browsing window, and login from there;