Are we talking about the Android app for Firefox? If so, all tabs are saved when swiping the browser closed. To close all open tabs at once, I suggest exiting the browser. To exit tap the three dots in the top right. Exit will be the first option at the top. If you go to settings, then tap Close Tabs, you can set your tabs to close automatically as well. If you like, you can set tabs to close after a day, week, or month. Please, let me know if this fixes your concern or if you have any questions.

You can check the startup setting.

• Options/Preferences -> General -> Startup: "When Firefox starts":
  "Show my windows and tabs from last time"
  "Show my homepage"
  "Show a blank page"

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

You can check if you have a user.js file in the Firefox profile folder that sets the browser.sessionstore.resume_session_once pref to true.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data