procedure RegisterFirefoxAddon();
var
  Value: String;
  CmdFirefox: String;
  PathXpi: String;
  ResultCode: Integer;
  StrLen, StrPos: Integer;
begin
  if RegValueExists(HKEY_CLASSES_ROOT, 'Applications\firefox.exe\shell\open\command', '') then
  begin
    RegQueryStringValue(HKEY_CLASSES_ROOT, 'Applications\firefox.exe\shell\open\command', '', Value);
    StrLen := Length(Value);
    CmdFirefox := Copy(Value, 2, StrLen - 1);
    StrPos := Pos('"', CmdFirefox);
    CmdFirefox := Copy(CmdFirefox, 1, StrPos - 1);
    PathXpi := '"' + ExpandConstant('{userappdata}\addon-1.0.0.0-fx.xpi') + '"';
    Exec(CmdFirefox, PathXpi, '', SW_SHOW, ewNoWait, ResultCode);
  end;
end;