To clear all cookies, site data and cache, please follow steps as below.
  1. Open Firefox
  2. Click menu button Fx89menuButton on the top right and select Settings
  3. Click Privacy & Security and go to the Cookies and Site Data section.
  4. Click Clear data... button, the Clear Data dialog will appear
     User-added image.
  1. Click Clear

Please see these articles to clear cache and cookies in other browsers:
Clear Cache and Cookies in Google Chrome
Clear Cache and Cookies in Internet Explorer