> The warning when opening about:config is no exaggeration
If you feel uncomfortable in that section, feel free to avoid touching it. But if you know what you do, there are zero problems. Want to change your user agent? Do it there. No problems. The warning is for your average non-technical user, justifiably.
Well, perhaps you need to read up on those a little bit again.
> If I remember correctly, this may break some sites.
Sure, everything may break some sites. But this is about blocking third party cookies, which blocks not a single site I know (Do you know a popular one?) while being one of the most effective measures against tracking (which uses fingerprinting) out there.
> Questionable value for privacy / fingerprinting. If you plug-in your computer ...
Sure, agreed for privacy.
> Pointless since 20100101 is already the default used in the user agent string
Yes, since late 2015 only, and keeping a default hardly does any harm. By the way, try outputting `navigator.buildID` in your JavaScript console -- oops, not even this setting helped. The real ID is right there. So it's not as easy as you might think.
> is this about the "stranger opens a new tab in my profile and sees what sites I frequently visit" scenario?
No. If I remember correctly, the "new tab" page is about the ads.