Alarm Clock от Michael Becker

Alarm Clock rings at a wall-clock time you set, or after a countdown you start — and it keeps working while you're busy in another tab, because the alarm lives in
the add-on's background page, not in the popup. Close the popup, switch windows, forget about it; it still fires.

Features

• Two ways to set an alarm — a wall-clock time ("07:45") or a countdown ("in 20 minutes"). A time that has already passed today rolls over to tomorrow.
• Rings with the popup closed. Every alarm is stored as an absolute timestamp and scheduled through the browser's own alarm API, so a throttled or skipped tick
  can only make an alarm late — never silent.
• Countdown badge on the toolbar icon: see how long is left without opening anything.
• Snooze or stop from the popup. Snooze is 9 minutes (5 in the Terminal theme).
• Several alarms at once, each one individually cancellable.
• Seven clock faces, each with a light and a dark mode: Basic LCD, Split-Flap departure board, Retro LED clock radio, Terminal console, Station railway dial,
  Nocturne luminous night dial, and Riviera mid-century shapes.

Privacy

No accounts, no analytics, no telemetry, no network requests at all. Your alarms and your chosen theme are kept on your own machine in the browser's extension
storage and never leave it. There are no content scripts and no host permissions, so the add-on cannot see the pages you visit.

It asks for exactly three permissions: alarms (wake up at the moment you asked for), storage (keep your alarms across browser restarts) and notifications
(show the alert when you're in another tab).

Open source: https://github.com/seebaermichi/alarm-clock