To disable this in firefox, just go to “Options->General Tab->Browsing and disable “Search for text when I start typing”. More info here . This is very useful in some cases, for example when you try to play WebGL games or when using pages like Gmail or Protonmail that have their own kb shortcuts.
UPDATED to version 73.0.1– In previous versions of Firefox this is in “Tools->Options->Advanced->General Tab” or in “Preferences->General->Browsing”