A common complaint with Firefox was that you couldn't sync between devices - even though that was perfectly possible! But apparently, people had a hard time finding the functionality.

You might not consider that use case important, but I wouldn't call it questionable.

Firefox sync is super convenient though, I don't really agree with you that it's a dark pattern. It does make using your browser more convenient across devices and I'm sure many people coming from chrome expect this to work out of the box especially since, AFAIK, google auto-logs you into Chrome as soon as it can.

On the other hand I find Pocket a much more questionable addition.