Tabs on top is the only location now, so there's no "option" or menu item for that any longer. Formerly the default was tabs below the Bookmarks Toolbar, with the option to have "tabs on top" in a menu, but now the user has to install an add-on to have tabs "below" the Bookmarks Toolbar.

If you do have tabs not on top, take a look at your installed add-ons and figure out which one moved the tabstrip - that will be where the "control" is located.

Many of us are using Classic Theme Restorer (Customize Australis) {web link} Restore squared tabs, appmenu, add-ons bar, small button view and more on Australis UI (Windows/MacOSX/Linux + Fx 29+).

This will restore many of the things the last upgrades took away.