User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:41.0) Gecko/20100101 Firefox/41.0 Build ID: 20151014143721 Steps to reproduce: Nothing... but I am guessing Sync "woke up" out of the blue Actual results: 2 unused extensions updated (ABP & DuckDuckGo) and my bookmarks got overriden by and old version (from 2/3 days, before I spent hours re-organising yesterday - yeah, my luck!) Expected results: To keep my bookmarks as they were.

Sorry for the delay, but it's hard to figure out what might have gone wrong here. Did the steps above involve multiple devices or just one? And did you reset your password at any time during this?

I'm sorry you had this trouble. We do know of a scenario where syncing an old device could cause this and we are investigating a solution in bug 1332290 - I suspect that's what you hit.

This does indeed look like an interesting demonstration of Bug 1332290 -- bookmarks don't have a TTL, so a node reassignment would be required, and indeed we see one. In this case the device presenting the log was node reassigned, was reassigned to an empty node, and filled it with its data. The clients count dropped from 2 to 1. That implies that perhaps another client then connected, downloaded the changed bookmarks, merged them with its own (missing tombstones, obviously -- and that's the bug) to yield an unexpected result, and uploaded the difference. One open question is why no syncing occurred for 2-3 days. The log seems to show a password change. That might be why...