I have Firefox set up to sync bookmarks and passwords between my laptop (primary) and desktop (the device I "add" after setting it up on my laptop). While my laptop never loses the settings, my desktop install will periodically just revert so that on the Sync tab, all you see is the "set up sync" button. It'll work for a while (hard to say how long, since it's not something I'd immediately notice) but I know it was working yesterday and today, reverted. Laptop and Desktop are both running 8.0, but this was happening in earlier releases too -- basically ever since the feature was embedded.