The need for registration had pissed me off several times. I love firefox and most of the extensions (add-ons) that are useful for me are available directly for download without registration. But there are several good extension which requires us to register to download it to Firefox. I am not alone. I saw a discussion on Reddit where many voiced similar complaints.
Just pop up a goddamn dialog: “This extension is experimental, use at your own risk, Yes/Cancel”.
I have to supply an email and confirm to download an extension? and by default you make that email publicly available? Great, Firefox has “protected” me from an extension, and made me vulnerable to spam! Smart thinking??? 
But I do agree with Mozilla on this one. While the dialog box solution would be perfectly suitable for advanced users, a large proportion of normal users wouldn’t even bother to read the dialog, and just click Yes. If their firefox then stops working (due to a buggy experimental plugin), they would probably have no idea how to fix it.
Having to register to use an experimental plugin is a much bigger roadblock, stopping unskilled users from accidentally shooting themselves in the foot. I think this is worth the inconvenience it causes to advanced users.
But they could have hidden something in about:config that advanced user could have enabled to download these extension without registration.