You can't because in their "wisdom" Mozilla decided to remove that (and many more great options) from their browser. I totally fail to see the reasoning behind it :(
Now you have to use an addon to add small buttons.
Check out https://support.mozilla.org/en-US/kb/how-to-make-new-firefox-look-like-old-firefox on how that overhaul the new look to the old look.

Here is a link to that Classic Theme Restorer extension:

• https://addons.mozilla.org/firefox/addon/classicthemerestorer/

You can check the settings of the CTR extension via its Options/Preferences button on the "Firefox/Tools > Add-ons > Extensions" page.

See also:

• https://support.mozilla.org/kb/common-questions-after-updating-to-new-firefox
• https://support.mozilla.org/kb/learn-more-about-the-design-of-new-firefox