The original tweets explained it. The user agent is impersonating a shared Facebook/Twitter/etc link preview/thumbnail. Many sites respond with markup more appropriate for a thumbnail when using that user agent, and if not, it’s unlikely that they serve anything worse, so it’s little harm done.
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The explanation for the retraction doesn't explain that user agent string. It's weird enough coming from macOS; it's even weirder coming from Firefox.
The behavior's still concerning in any case, whether it's macOS silently doing it or Firefox silently doing it.
The original tweets explained it. The user agent is impersonating a shared Facebook/Twitter/etc link preview/thumbnail. Many sites respond with markup more appropriate for a thumbnail when using that user agent, and if not, it’s unlikely that they serve anything worse, so it’s little harm done.