Saved passwords cannot have their corresponding website edited.
When opening the firefox nightly password manager and selecting a password, you have the possibility to edit your saved password by clicking the edit button. After clicking it, you are able to modify the username and the password itself. However, you cannot modify the url/website this password was saved from. For example, I manually entered the login and password for, let's say, my reddit account. However, I misspelled the website and typed in reddot.com. Now, I cannot modify this passwords corresponding website and have to delete it, and make another one where I don't misspell the url. If this is the case for technical reasons/limitations, then I totally understand. However, if this is the case because it was overlooked, then I'd be looking forward to seeing it in future updates.