GMarks helps you sync & manage your bookmarks with Google Bookmarks with included “sidebar, a toolbar, a quick search with Home+Home similar to Google Desktop's Ctrl+Ctrl search, and full content search of your bookmarked pages.”
GMarks supports nested labels with a customizable separator. It has a toolbar button which turns yellow when the page is bookmarked and when middle clicked can add/remove the site. GMarks also adds a tab to the Firefox add bookmark window to add bookmarks directly to the Google Bookmarks site.
Also included are filters, which can be used for both batch editing and for automatically adding descriptions and labels to new bookmarks, and an Organize GMarks window with inline editing of bookmarks.
GMarks allows for easy migration to and from Google Bookmarks. You can import your Firefox bookmarks to Google or export your Google Bookmarks to a bookmarks.htm file. GMarks is completely separate from your Firefox bookmarks and does not remove or hide them.