Bookmark synchronization has always been something I’ve wanted. The closest match I ever saw to what I wanted was the Yahoo! Toolbar. Unfortunately, the Y! Toolbar doesn’t have support for tabs in Firefox (I have a folder of bookmarks that I open every morning using the Firefox “Open in Tabs”).
Today I’ve started using the Firefox bookmark synchronization extension. I’m already pretty happy with it. It integrates with Firefox’s built-in bookmarking mechanism, meaning the tab integration works automatically. Additionally, it set up my bookmark toobar correctly (I put my most frequently accessed bookmarks there). It even preserved the “shortcuts” I set up (bookmarks with the keyword attribute set).
I’m currently storing my preferences in a WebDAV store and fetching them over HTTP. I may plunk down the extra cash to get my DreamHost account set up with a static IP so I can get an SSL certificate. That way I can start doing WebDAV over HTTPS. If you don’t have a WebDAV store, you can use FTP instead. There’s currently no support for SFTP (bummer), but maybe it’ll be there one day. If I end up getting WebDAV with SSL, though, I really won’t care.