5 Lessons I Learned Building a Firefox New Tab Extension from Scratch

I spent the better part of a month building Weather & Clock Dashboard — a Firefox new tab extension that shows live weather, world clocks, and a search bar. No frameworks, no bundlers, just pure HTML/CSS/JS.

Here's what I wish someone had told me before I started.

1. The manifest_version: 3 Transition Is Mostly Smooth, But Watch for Service Worker Gotchas

Firefox now supports Manifest V3, and I decided to go all-in. Most things just work — declarative content scripts, the new permissions model, cleaner background handling.

But service workers have a critical limitation: they go inactive after 30 seconds . For a new tab extension that needs to refresh weather data periodically, this matters.

My solution: fetch weather data on each new tab open, not on a persistent background schedule.…