Xmarks not syncing between Firefox and Safari

I have a macbook Air running mac OS 10.5.8 (Leopard) and a macbook running mac OS 10.4 (Tiger). I use Firefox v.3.5.3 and Safari v.4.0.3 browsers. I had initially installed Foxmarks using Firefox and it worked fine. To get my bookmarks to also sync with Safari, I installed Xmarks for Safari. When I change bookmarks on Firefox, then click sync, it opens up and appears to be syncing with Xmarks for Safari but the change does not show up in Safari or in My Xmarks online. If I change a bookmark in Safari, it does not update in Firefox. Firefox is my primary and preferred browser. How can I get the bookmarks linked on both browsers?