Might try right-clicking in the tab bar area (not on top of a tab) and select Customize Toolbar to see if the "+" button was put back into the collection of available items to add.

If so, drag it back onto the toolbar.

If missing items are in the Customize palette then drag them back from the Customize page to the toolbar If you do not see an item on a toolbar and in the Customize palette then click Restore Defaults in the Customize palette window to restore the default toolbar set.

• https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars