Rollback Whatsapp Web update?

A few days ago this message popped up in my Whatsapp web (in Firefox browser) and I clicked on it to update my Whatsapp web.

Now I wish to rollback the update to the previous version. Anybody know how I can do this? I guess the update must have loaded some new script or code in my PC, can I just delete those scripts/codes?

FYI, the PC runs on Win XP SP3 and I'm using Firefox 52.9.0.

Thanks.