Firefox desktop prompts for login details as well, it seems to also do so on a couple other forms for me, such as Special:BlockList, Special:ActiveUsers, Special:ListFiles, Special:GlobalRenameRequest.

Is this still happening? The field has autocomplete="off" so I wouldn't expect it.

Yes, the browsers ignore autocomplete when determining if a form is a login form.

They certainly don't ignore it; e.g. Chromium advises autocomplete="username" for login forms. But I'm sure they have some other heuristics, I just don't see what it could be in this case - the form is not labeled as having anything to do with usernames. The field name is target, the ID is ooui-php-1, the form target is Special:CentralAuth the only even remotely relevant-sounding classname is mw-widget-userInputWidget on the parent... I guess the form label is Username: but I doubt we want to change that just to confuse browsers with stupid heuristics. Some people claim setting an invalid autocomplete value helps, I guess we could try that.