It depends.

Firefox's built-in favorite sites list appears on the Bookmarks (Alt+b) menu or in the Bookmarks sidebar (Ctrl+b). Is that what disappeared?

Some users also store favorites in a toolbar, such as the Yahoo toolbar.

Some users also use an add-on named PlainOldFavorites which displays the Favorites menu from IE directly in Firefox.

Could you describe what's missing in a little more detail?

Are you missing bookmarks or are you looking for a way to access the bookmarks?

If you have problems with bookmarks and history not working properly then check for problems with the places.sqlite database file in the Firefox Profile Folder.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

If you lost the Bookmarks Toolbar then make sure that you've set that toolbar to display.

If the menu bar is hidden then press the F10 key or hold down the Alt key to make the menu bar appear.

Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible:

• "View > Toolbars"

Open the Customize window to set which toolbar items to display:

• View > Toolbars > Customize
• Firefox > Options > Toolbar Layout

• Check that the "Bookmarks Toolbar items" is on the Bookmarks Toolbar
• If the "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette in the customize window to the Bookmarks Toolbar

• If missing items are in the toolbar palette then drag them back from the Customize window on the toolbar
• If you do not see an item on a toolbar and in the toolbar palette then click the "Restore Default Set" button to restore the default toolbar set up

• https://support.mozilla.org/kb/Back+and+forward+or+other+toolbar+items+are+missing