Normally you will see three buttons along the top of the menu panel (the panel that opens when you click the "3-bar" button). They would look something like:

- | 100% | +

These are a unit, so you can right-click any part of that and Move to Toolbar. Or to drag and drop it to a specific place on the toolbar, you can first use Customize. This article has details on that: Customize Firefox controls, buttons and toolbars.

Success?

Note: the center button shows the current zoom level, but when you click it, it resets. There isn't a built-in way to type the percentage you want.

• if missing items are in the Customize 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 Customize palette then click the "Restore Defaults" button to restore the default toolbar setup