[thesuitonym]

Not sure if this is what the GP was referring to, but I constantly find Google and Amazon (two sites I almost never use) pinned to my frequently accessed sites in Firefox. I unpin them, then check all settings for any kind of advertising opt, and yet, some month later, they're back.

[bentcorner]

Are you installing Firefox and signing into them on new machines? I don't have hard evidence but I feel like when I install FF on a new box and sign in, the pinned sites sometimes show up on other machines when everything is synced.