With all the account hacking that seems to be going on, I wont spend a minute checking any add-on that is required to access/bypass the credentials on my account, whatever the disclaimers say. Until the Forums comes up with an official app (if it does) I certainly will not risk my account.
when it said "Access your data for.... " it doesn't mean or need your credentials information but "data" mean here is html code you are currently viewing, you don't need to login to use this addon. FYI all addon that interact with website like "youtube downloader addon" need same access but it not mean the addon need your YouTube account information.
It open source, people can just download the code, edit whatever he like and upload to mozilla for signing and use it.