Background Script errors on Firefox

[ma-anwar]

Steps to Reproduce

1. npm install
2. npm run dev:firefox

Now add the extension in about:debug. Click inspect, the following error message shows up

Uncaught TypeError: browser is null
    <anonymous> browser

-polyfill.js:1085
    <anonymous> browser-polyfill.js:1157

The content script, popup etc all work. However sending any message to the background script does not work

firefox.zip