Are you referring to the menu bar? If so, see Restore the Menu bar in Firefox

If you actually want bookmarks in a tab, put this into the address bar:

chrome://browser/content/bookmarks/bookmarksPanel.xul

My bookmarks tab has disappeared, also. I still have the menu bar. Read the problem!

Bookmarks doesn't show in a Tab - the OP didn't describe his problem in a manner that Firefox support helpers understand, so Chris was guessing about the problem, and Anonymous, likewise was guessing about the OP's problem.

If the Menu bar is showing and the word Bookmarks doesn't appear on the Menu bar, something you have installed is blocking that Menu item from being shown. IIRC, one the the Delicious extensions (and maybe one other bookmarks sharing extension) has an option for hiding the Bookmarks Menu bar item. Look at the options for the extensions you have installed and see if you have that preference selected.