There is no "master" or "slave", devices are equal depending upon the user settings for types of data to be synchronized. The Sync server sits in the middle, receives data from each of the devices & merges that data, and then sends that data that is 'missing' to the appropriate device. The server synchronizes the selected data across all devices, per the settings on each device. And when you're editing or re-arranging bookmarks and moving a massive number around, don't have Firefox running on more than that one device. If Firefox acts like it doesn't like those changes ''["then my editings switch back"]'' -> Disconnect that device from Sync until you're done editing / re-arranging. ''Firefox will send 'changes' to the Sync server one at a time and it seems that at "some point" it will start 'collecting changes' and sending them in batches to the Sync server. And from my observation that is where Places database corruption begins when dealing with desktop / laptop Firefox.'' ---------------- As far as duplicated bookmarks data - do you have a "mobile" device running Firefox for iOS or Firefox for Android connected to that Sync account? If so, that might be the cause. ''Let us know here and we can then advise you how to proceed,'' ------------- If not, disconnect all devices from Sync using Options > Sync --> Disconnect button. Then run Places Maintenance - https://addons.mozilla.org/en-US/firefox/addon/places-maintenance/ - to fix the Places data base on each device. Once that is done, reset your Sync account password to clear your data from the Sync server. Then connect each device to Sync one at a time, allowing Firefox to be running for a few hours or so on a device before connecting the next device. ''Variables include the amount of data Sync needs to deal with, the 'load' on the Sync servers, and 'congestion' / 'speed' on the internet at the time Sync is transferring data.''