For OS X users that use both Safari and Firefox, Xmarks needs to be setup on both browsers separately. To install the program for use with Safari, download Xmarks for Safari from their downloads page. Following installation, open the settings page by clicking on the Xmarks icon in the menu bar and selecting "Xmarks for Safari Settings". Click "Run Setup Wizard" to create an account and to start syncing your bookmarks.
For Firefox users not using Safari, download Xmarks for Firefox from the download page to setup and begin managing your bookmarks. Those that use Safari and already downloaded Xmarks for Safari can simply install the Firefox plugin to sync the bookmarks from Safari.
In addition to backing up and syncing bookmarks, Xmarks also has more advanced features like creating profiles to have different bookmarks in different browsers and a search tab for improving your Google search results. More information about Xmarks and these advanced features are available at the Xmarks website.