Fortunately, my quest has ended in success with the Bookmarks Synchronizer. This slick little Firefox extension enables bookmark syncing via any good 'ol fashioned FTP/WebDAV server, and for the past couple of days it's been working like a charm. You can set the extension to sync at the startup and/or shutdown of Firefox, which also works really well with one minor catch: quitting Firefox causes this extension to kick into syncing action, which effectively stops Firefox from actually quitting so the sync can finish. Once a small progress window disappears, signalling a successful upload, you can then truly quit Firefox.
I'm using Firefox 1.07 and the extension works just fine. The Mozilla download page makes no mention as to whether Bookmarks Synchronizer works with the 1.5 beta, so you're on your own there. But if you've been looking for a way to cut down on computer-induced headaches and an easier way of keeping all those bookmarks n-sync, (hah!) I definitely recommend downloading Bookmarks Synchronizer.