Go to the Menu Bar and click "History" . If you want to clear recent histories then click "Clear Recent History" . But if you want to clear all the histories then click "Show all History" then choose anyone of all the histories and press ctrl+A and then press Delete .

Tap the Menu button > Settings (you may need to tap More first) > Privacy > Clear private data.

See How do I clear the history?:

Debapriyo Bhattacharya this is about mobile Firefox for Android and not on a desktop OS.