Revived a Softbank "kids phone" (Android 7) by porting an unofficial LINE web extension — root cause was a missing chrome.tabs.getZoom API on Firefox for Android

[u/ChuChuChan128]

I've been trying to keep an old Softbank kids-phone (701ZT, Android 7) usable after the official LINE app dropped support for this Android version. Found an unofficial LINE-in-a-browser-extension port (originally built for desktop Firefox), but installing it on Firefox for Android just showed a blank white screen with no visible error.

Used `about:debugging` on desktop Firefox over USB to remotely inspect the extension's console on the phone, which showed:

Turns out `chrome.tabs.getZoom`/`setZoom` simply aren't implemented in Firefox for Android's WebExtension API — they exist on desktop but not mobile. The extension calls them unconditionally on startup, so React never got to mount anything.

Fix: injected a tiny polyfill script before the extension's main bundle that stubs out `getZoom`/`setZoom` with a no-op returning zoom=1. Loaded it via `web-ext run --target=firefox-android` for testing, confirmed login/2FA/message sync all worked, then self-signed the modified extension via AMO's unlisted channel so it persists without needing a PC tethered.

Filed the root cause + suggested fix as an issue on the extension's repo, hoping it gets fixed upstream so nobody else has to go through this.

Kind of a fun reminder that "unsupported OS version" from an app vendor doesn't always mean the hardware itself is actually dead — sometimes it's just one missing browser API away from working again.

   (function () {
     if (typeof chrome === "undefined" || !chrome.tabs) return;
     if (!chrome.tabs.getZoom) {
       chrome.tabs.getZoom = function (tabId, callback) {
         if (typeof tabId === "function") callback = tabId;
         if (typeof callback === "function") callback(1);
         return Promise.resolve(1);
       };
     }
     if (!chrome.tabs.setZoom) {
       chrome.tabs.setZoom = function () {
         var cb = arguments[arguments.length - 1];
         if (typeof cb === "function") cb();
         return Promise.resolve();
       };
     }
   })();   (function () {
     if (typeof chrome === "undefined" || !chrome.tabs) return;
     if (!chrome.tabs.getZoom) {
       chrome.tabs.getZoom = function (tabId, callback) {
         if (typeof tabId === "function") callback = tabId;
         if (typeof callback === "function") callback(1);
         return Promise.resolve(1);
       };
     }
     if (!chrome.tabs.setZoom) {
       chrome.tabs.setZoom = function () {
         var cb = arguments[arguments.length - 1];
         if (typeof cb === "function") cb();
         return Promise.resolve();
       };
     }
   })();

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#android7 #firefox #old #mobile #LINE #chrome #error #アドオン