I've had the same problem with the backspace. When hitting backspace while editing, often (not always) the browser goes to previous page (in history), so I lose my edits. Going forward does not give me back my edits.
If this poses enough of a problem, you might consider disabling the "backspace navigates back" behavior by setting browser.backspace_action in about:config to 2 and just using the shortcuts with modifiers (Cmd+[ or Cmd+Left, et al) or three-finger swipe. This is actually the default way interaction works on Linux builds.
This has been the source of much pain on my part on more than one occasion.
It should only go back to previous page if the input is not focused. I've been bitten by this myself a few times and have developed a habbit to check if some input is focused before pressing backspace.