I bet that’s because people trying to type into textbox would sometime defocus, hit back and lose all their text. At least back should go back if there’s no data to lose in the tab !

Apparently you can get backspace back with

https://addons.mozilla.org/en-CA/firefox/addon/backspace-enabler/

I also just gestures to go back with

https://github.com/Robbendebiene/Gesturefy

You draw a leftward line with rightclick and it goes back