Update: My conclusions were wrong. The sync workaround doesn’t work. See end.
I’m using Firefox Sync on 3 systems. Recently I added some bookmarks on one of the systems that were not propagated to the other systems. At first I checked FireFox support and there is a long list of things to check. After checking a few things, I decided to do a test rather than going through the rest of the checks on that page. I did disable the plugins – long lists of Java Consoles of various versions – and that made no difference. What I found out when I did a test was that some changes are propagated and others are not propagated. Here is the test that I performed and here are pictures of the results.