Oh—I found confirmation of my Mozilla bug: not only it’s confirmed, but it’s even intentional. Mozilla wantingly disobeys the DNS standards to avoid a vague security risk that I won’t comment, because I’d have to document myself enough to form an opinion. Anyway, there’s a workaround:
For now you can “un-pin” addresses by going offline and back on (via the file menu or the plug icon in the status bar) (oh, so that’s what this icon is for).
So, if you intend to access a site through dyndns.org (or similar) and it quits responding, try this method. I haven’t had an opportunity to test it yet, but it seems sound. And at least it’s simpler than restarting Mozilla.
P.S. A dozen hours later, I can verify it. Just double-click the icon in the lower right corner, and the dyndns sites work again. To think that this bug has been know for something like six months…