hello moscovit, there is no explicit "register" button.
if you want to sign up as a contributor, you can do so trough https://support.mozilla.org/get-involved - if you just want to ask a question in the support forum (as you have just done), you have already registered through the username/email you have provided there...

From what I saw, the only easy way to register is to go through the process of asking a question. At some point part way through that, you'll be asked to register.