To delete Firefox bookmarks, use the Library window where all of your bookmarks are organized in folders in the left menu. Your browsing history is also available in this same window, so you can delete this data, too.
Delete a single bookmark the same way you add a bookmark. This is the quickest way to delete a bookmark when you happen to be on that Web page. Click the star-shaped Bookmark icon beside the the address bar and then click the Remove Bookmark button.
Click the Bookmarks menu button and select Show All Bookmarks to open the Library window containing your bookmarks.
Open the Bookmarks Menu folder by clicking the small arrow beside it and then select Recently Bookmarked. Unless you're a bookmark power user, this should display most of your bookmarks on the right. If it's not there, expand the All Bookmarks folder and then look in each sub-folder.
Right-click the bookmark and select Delete. To select all of the displayed bookmarks, click one and then press Ctrl-A. You can also Ctrl-click bookmarks to select several at once.
Click the History folder in the left menu if you want to delete your browsing history. Click any item on the right and press Ctrl-A to select all. Right-click a highlighted item and select Delete.
Double-clicking any month or day opens that folder if you want to delete only specific items in your browsing history.