GMarks includes a 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 in FF2 to add bookmarks directly to the Google Bookmarks site. There are several other toolbar items which can be used for directly adding bookmarks to GMarks. Along with using the start toolbar item to open the sidebar you can open it from the View->Sidebars->GMarks menu or the shortcut combo Alt+M.
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.
For support or questions go to http://groups.google.com/group/GMarks