At https://discuss-space.wmflabs.org/, clicking "Sign up" or "Log in" starts the authentication process with Phabricator correctly. With some versions of Chrome and Firefox, users can create their new account without problems.
However, there are problems with at least some versions of Safari and Firefox - it enters a loop after clicking on "Continue to Application", giving a brief flash of the Discourse registration screen, but then returning to the Phabricator page.
Let's investigate. If you are experiencing problems, please list your browser version below.
If you are able to create and account and log in, please add your browser version too, so we can understand better the problem.