On Firefox for Android on my phone, with the native keyboard the button to hide the keyboard is usually replacing the back button. With your own keyboard, the button to hide the keyboard is on the top right, but my muscles memory goes to hit the back button (which brings me back to the previous page rather than hiding the keyboard).
I'm not sure if that would be worth it to add an history element so that the back button can hide the keyboard, but for me that would make things more natural.
Thanks for the feedback! Yeah I wanted to create a custom keyboard because I felt that the native keyboards were missing some buttons that were useful like the +/- button. I don't want to take away from the native experience so thanks for letting me know.