Hi Kerr, add-on authors have the choice of storing data either in "local" storage (which does not sync) or "managed" storage (which does sync). Sometimes they give the user a choice in their settings.

The reason authors don't just use managed storage for everything is that it is capped at a small amount of data, and add-ons break as soon as that amount is exceeded. Hopefully Mozilla can increase the cap in the future, but understandably, Sync isn't going to be able to handle unlimited data.