The Mozilla website has several separate sites within it. You will need to create a user account for each.

An account on support.mozilla.org is only for this site. For addons.mozilla.org https://blog.mozilla.org/addons/2016/02/01/firefox-accounts-on-amo/ https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer The Addons.mozilla.org site used to share account with its a.m.o support forum when it was at forums.mozilla.org (had some issues) until amo support forum was moved to https://discourse.mozilla-community.org/c/add-ons