We've been getting reports of some duplicates and bookmarks moving between folders over the last few months. We've been actively trying to track down this bug, but it's been sufficiently rare that we've been unable to catch the problem in users' log files, and we've never had any success reproducing the problem in-house. That is, until late last week, when we finally tracked it down! Needless to say, it's a lot easier to find and fix something when you can make the error happen in the first place. So after some swift QA, we're releasing this version today (3.4.10 on Mozilla's Add-On site, and 3.4.11 on the Xmarks site) that contains the fix. (Only Firefox users are affected, so no need to worry if you are a Safari, IE, or Chrome user.)
A big THANK YOU to all of you who experienced the problem and were willing to work with us to get to the bottom of it! You've been very gracious and patient, and we are grateful for your support and effort.
We're also working on a major release that should be coming up in the next few weeks, but we didn't want to wait to get this important fix out. So hopefully you'll forgive the frequent updates when the next one comes out soon!