If you doing a very long run, then it is recommended to do the “export to file” for backup purposes, periodically. Save the resulting JSON file to some safe storage (google drive, dropbox, etc).
What is your browser, btw? Google Chrome is highly recommended, and Firefox is also a good choice. I have not tested the game with other browsers, so can’t predict their compatibility.
Do a quick test: start a new game, exit the full-screen mode to see browser menus, and click the “export to file” button. Then click “show downloads” or just navigate to “chrome://downloads/” (on Chrome). If there are no files, I will be really surprised.