In most sources, Zotero automatically detects when you are looking at information for a book, article, or other source in Firefox and inserts a "save reference" icon right into the address bar. The address bar icon will be a book, document, newspaper, etc. To save, simply click the icon. If you are looking at a list of references, you may see a folder icon. This icon will prompt you to select multiple items from the list to save.
If Zotero does not automatically detect (provide a "capture icon" for a list, book, or article) to import a reference into your library you can use the export feature provided by the database.