The problem is because you tripped the Caret Browsing feature at some point, and accidentally hit Yes.
To fix this, you’re able to just hit the F7 key again.
Remember, you’re free to double-select any boolean value to toggle between true and false.
Fix Firefox Not Scrolling with Up/Down Arrow Keys or Home/End Keys
If you've encountered a problem where your Firefox installation no longer scrolls when you use the up or down arrow keys, and even the Home or End key
