I finally did the search for how to disable the behavior in Firefox where clicking on Backspaces triggers a ‘Back’-event in the browser.
According to https://support.mozilla.org/sv/questions/924490:
“You can set the Integer pref browser.backspace_action to 2 on the about:config page to disable the backspace action. BTW Shift + Backspace does the reverse: going Forward if possible, so that is taken as well. ”
It works great, I should have done this earlier to prevent all the frustration of breaking web forms.