Over the last few days I've read at least a dozen blog posts and tweets from people who are confused by one of the changes at AMO.
No, Mozilla has not switched from anonymous add-on installation to requiring a login.
What's new is that we now expose sandboxed (not ready for prime-time) add-ons in search results along with fully released add-ons.
The experimental, sandboxed add-ons have always required a login. There's no change here regarding when a login is required. We're just making the sandboxed extensions more visible.
Regular users do not need an AMO login to install release version, Firefox compatible extensions.