Helpful tip: there's a setting in Firefox to block all notification requests. It's under Settings > Privacy & Security, then scroll down to the Permissions heading. Click the "Settings..." button next to the the Notifications entry and tick the box for "Block new requests asking to allow notifications".

I assume there's an equivalent in Chrome, but I don't know what it is off the top of my head.

Ninja edit: Removed my attempt to hyperlink directly to the relevant Firefox settings page because it wasn't working.