How do I Clear the cache and cookies ?

You can clear them by clearing your browsing history:

https://support.mozilla.org/en-US/kb/clear-browsing-history-firefox-ios

The good news is, every single time the app launches to the purple empty home screen, all your cookies and caches are already cleaned out.

If you want to do this manually, just press the trash can icon — it will close your current tab and wipe the whole browser clean for a fresh start.