Firefox for Android does not normally have a Quit button on the menu. Instead, it follows the platform guideline to let users quit an application by swiping it off the recent applications list. If you enable the feature to delete data when you close Firefox, Firefox will add a Quit command to the menu. For example, you could select Cache to clear just cached files when you close Firefox. This article has the info: [[Clear your browsing history and other personal data on Firefox for Android]] (the "Delete data on exit" section).