User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:55.0) Gecko/20100101 Firefox/55.0 Build ID: 20170407030233 Steps to reproduce: I have two computers both with Firefox and Sync, my main PC that I always use and a second one that I rarely use so when I turn it on it needs to update and that's what it does... Actual results: BUT Sync installs me some old modules that I tried once and deleted right after, it seems that when I install something it's pushed to my sync account and even if delete it minutes later it still stays here AND the secondary PC then sync itself which brings me back some old bookmarks and maybe settings? on my main PC... Expected results: I'd like that when I turn on my secondary PC after a long time it only updates in one way to make it look like my main PC.

Were sorry you are having this problem. We've identified why this happens, and that it only happens when there's a particular series of events, but unfortunately we had this series of events over the last few weeks. We are still working on a fix here via bug 1332290, so I'm making this a dupe of that. Again, apologies for the inconvenience and thanks for taking the time to open this bug.