I wanted to follow on from the likes of: https://support.mozilla.org/en-US/questions/1220898 - the situation where you turn on some very old device that hasn't synced in a long time, and the data on it is so old that Firefox Sync decides to merge it with your latest sync data, instead of replacing it with the latest sync data Unfortunately an avid Firefox user, if they find that they've turned on an old device and its polluted their sync with stale data, will find a bigger problem than that post suggests. I have Firefox and Firefox Sync installed/setup on a number of PCs and Phones. I turned on an old machine, I think in July 2020, and polluted my sync with old bookmarks. Now Marc had some good advice here: https://support.mozilla.org/en-US/questions/1212089#answer-1099270 > For what it's worth, another reliable way to remove all data from the server is to visit about:preferences#sync, disable one of the data-types (say, bookmarks) and perform a manual sync - all bookmarks will then be removed from the server. Re-enabling and re-syncing will re-upload all your local bookmarks So I found this process worked to restore a previous good set of bookmarks before the polluted sync: 1) on any PC, perform the above to remove the online sync 2) disable all syncing on that PC 3) restore previous known-good bookmarks 4) re-enable syncing But then, any one of your other devices, which all synced the polluted sync, will just sync the polluted sync back to the server again, which will then sync it back to all devices: so my polluted sync returned again, everywhere. In particular, I found that any phone you have syncing will keep the bad sync running and cause it to be re-synced everywhere, until you uninstall Firefox from that phone. So the following is how I finally managed to return to my pre-bad-sync state (which had to be done in exactly this order): 1) uninstall Firefox on any phone 2) disconnect any PCs from the internet 3) on your 'newest' PC, do the above 4 steps to restore good sync on that machine and to the online sync server 4) on any other PCs, copy the JSON bookmarks file you used in the last step to those PCs and restore it to those also, reconnect them after (since they will no longer pollute the good online sync once their data has been made good) 5) reinstall Firefox on all phones and set up each to sync again As you can imagine, this was quite time consuming to get right, and I now live in fear of turning on an old PC (in particular we don't know HOW stale an old sync has to be to cause this problem, so its a constant worry!). And note that bookmarks was all I cared about: I don't know what I would have done if I also cared about other sync data This is a complex issue, hence I want to limit what I'm asking here. I just want to know the best/easiest way of avoiding this happening again. How about this: If we are, perhaps, safe to assume that any PC which has synced in the past year is 'safe' from this problem, maybe the easiest solution is to treat any PC which hasn't been turned on for longer than 1 year with kid gloves? If so, what's the simplest thing to do when turning on any such PC? The first step is surely to disconnect it from the internet before Firefox starts up, and then what? (remove Firefox entirely? restore good JSON? delete user profile? etc etc - I'd like to know what are the exact steps I take, in order, so maybe a bullet list (I have both Macos and Windows machines, and an Iphone and an Android phone). I'd also like to know if this issue is likely to affect phones in just the same way: say I turn on an old phone because I have to take my new phone to be repaired: is this just as dangerous as turning on an old PC? I'd welcome any other thoughts on this issue also of course. (Note I haven't shared data about my OS/Browser etc since by its very nature this issue is cross-platform and spans numerous versions of FF)