If your bookmarks have suddenly disappeared, don't worry - you should be able to recover them. This article explains how to get back your bookmarks if they disappeared unexpectedly.
If you were using the Bookmarks Toolbar for quick access to your favorite bookmarks and the toolbar is now missing, you may have turned off the option to display the Bookmarks Toolbar. To turn it back on:
For more information, see the Bookmarks Toolbar - Display your favorite websites at the top of the Firefox window article.
To recover bookmarks that were previously saved in the Bookmarks menu or on the Bookmarks toolbar but are now missing, see Restore bookmarks from backup or move them to another computer.
To access your saved Google Bookmarks, go to www.google.com/bookmarks and log in. Once there, you can export your Google Bookmarks and import them into Firefox if you wish. For more, see Google Toolbar is no longer compatible with Firefox - Here are some alternatives.
If you have restarted Firefox after installing an add-on and your bookmarks are gone, it's possible that Firefox did not close properly before restarting itself. To recover your bookmarks, restart your computer.
If you have imported your bookmarks from another browser, they may appear in the Bookmarks menu in a specific folder. To find your imported bookmarks, do the following:
If desired, you can move your imported bookmarks to other folders. For more information, see Create bookmarks to save your favorite webpages.
Your bookmarks are associated with the profile that you are using. For most Firefox users, only the default profile is used. There may be instances when you wish to create a new profile in Firefox. When this new profile is activated, it contains the default set of bookmarks which may give you the impression that you have lost your personal bookmarks. This troubleshooting sequence helps you to identify if a new profile has been created and how to recover your bookmarks from the previous profile.
Based on information from Lost bookmarks (mozillaZine KB)