If a hacker exploits a vulnerability in Firefox or Chromium Legacy, they could potentially escape the browser sandbox and take over your High Sierra machine, because the OS itself has unpatched kernel flaws.
To use WhatsApp on Mac OS High Sierra, you'll need: whatsapp for mac os high sierra
Because you are on High Sierra, you lose out on some modern conveniences even via the browser: If a hacker exploits a vulnerability in Firefox
Have a different experience? Found a working native installer? Share your findings in the comments (but remember: never download WhatsApp installers from third-party torrent sites – they are 99% malware). ranked from best to worst.
Just because the native app is dead does not mean you cannot use WhatsApp on your High Sierra machine. Here are the three reliable methods, ranked from best to worst.