Xmarks wrongly syncs ISP a/c credentials even when told not to

Yesterday I was working on a friend's (Windows 7) PC, where I have my own (admin level) user-account. We both use Firefox 3.5.3 with Xmarks 3.3.2. I imported all my bookmarks and passwords to my Firefox profile on her PC using Xmarks. (This was the first time I had done this on a PC that was connected to the internet via a different ISP to my own.) When doing so, I was careful to specify that the bookmarks and passwords on the local PC should be discarded and overwritten by the data stored on the net. Also, in the Xmarks sync-options settings on my friend's PC, I was careful to disable automatic sync'ing and specified that before sync'ing on shut down, Xmarks should ask me first. Despite these precautions, I discovered this morning that this had created an unusual problem for me when using Firefox on my own (Windows XP) PC.

Both my friend and I use the Firefox extension Net Usage Item (ver 1.2.236.1). It displays a meter on the browser's toolbar showing how much of my monthly ISP-imposed download quota is currently used up. However we use different ISPs for our internet connections. This morning I noticed an error alert in my Net Usage meter telling me to check my settings because it could not log on to my ISP account to retrieve my usage stats. I noticed that in my Net Usage settings, my username and password had been replaced with my friend's logon credentials for her ISP. (Only those two fields had been changed BTW; the ISP field was still showing my own ISP's name. If that had been changed as well, it would probably have functioned correctly, but it would have shown me her usage stats instead of my own.)

The only way this could have happened is by Xmarks syncing her data with mine, but the precautions I had taken above should have prevented this. How can I ensure that this doesn't happen again?