Firefox Custom Icons - Some of the latest icons from our Firefox icon system. 🦊 We’ve been working to expand our system with over 300 icons — new icons and sizes are based on the key visual rules I helped define last year.Firefox Custom Icons - Some of the latest icons from our Firefox icon system. 🦊 We’ve been working to expand our system with over 300 icons — new icons and sizes are based on the key visual rules I helped define last year.