My first Firefox add-on was a manifest change

KH4 Companion is a small extension I built: it counts down to Kingdom Hearts IV, puts the days remaining on the toolbar badge, pulls series news and trailers from public feeds, carries a lore compendium, and hides a three-lane rhythm minigame in the popup. It has been on the Chrome Web Store since 19 August. As of this week it is also on addons.mozilla.org , which makes it my first Mozilla listing. I had been putting the port off, because "port" sounds like work. It was not. Same build, same version number, same feature set — what changed was four keys in manifest.json . This is the writeup I wanted to find before I started. The thing nobody tells you first The blocker is not your code. It is that AMO rejects the package before it ever shows you a listing form. So the order of operations is: fix the manifest, get the linter to zero errors, then worry about icons and screenshots and copy. Assets built against a package that cannot upload are wasted. npx addons-linter@latest <extension-dir> is the gate. Run it before you touch anything else. 1. Firefox needs an explicit add-on ID Chrome derives an extension ID for you. Firefox does not — in MV3 you must state it: "browser_specific_settings" : { "gecko" : { "id" : "kh4-companion@dhseadev.online" } } The email-ish form or a {8-4-4-4-12} GUID both work. Pick carefully: this ID is your update identity forever. Changing it later means a new listing, not an update. 2. There are no extension service workers in Firefox This is the real difference, and it is smaller than it sounds. Firefox runs an event page where Chrome runs a service worker. background.service_worker is simply ignored, with a BACKGROUND_SERVICE_WORKER_IGNORED warning. The cross-browser answer is the dual key: "background" : { "scripts" : [ "core/lib.js" , "background.js" ], "service_worker" : "background.js" } Chrome reads service_worker . Firefox reads scripts . One file, both browsers. Two traps live in here, and both pass a manifest review and fail at run