I saw this question during the stream, but didn’t get a chance to answer. Continuing the discussion from July Livestream Q&A - Patreon & Forum Priority Thread! 
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Private windows and guest windows provide the same privacy protections, the difference is that private windows have access to your browsing history, bookmarks, and (sometimes) extensions; whereas guest windows do not.
For example, if I opened a private window and typed discuss. in the address bar, it would autofill in discuss.techlore.tech because this forum is in my browser history in my regular profile, even though the private window doesn’t save history itself. If I opened a guest window and tried the same thing it wouldn’t autofill anything, because guest windows are disconnected from the regular profile entirely.
You can’t edit the defaults of a guest window, because the whole point is that it isn’t tied to a browser profile at all, but if you just want a private window with your defaults already set, that’s exactly what regular private windows are for in the first place. Guest windows are basically just for letting someone use your browser without letting them see your bookmarks or history, it’s a pretty niche use-case.