Firefox follows Google's Android user interface guideline of getting rid of a Quit or Exit button. However, you can get it back in either of two ways:

(1) Set Firefox to clear at least one category of personal data at shutdown. The steps for that are in this article: Clear your browsing history and other personal data on Firefox for Android (the "Delete data on exit" section toward the bottom)

(2) Install this add-on: https://addons.mozilla.org/android/addon/quitnow/

I don't know whether anyone has created a similar add-on to restart.

I forgot to mention: the Google-prescribed method of closing an application is to display the running program list using the dedicated button and swiping applications left or right to close them.