I had updated the package using DefinitelyTyped Firefox WebExt Browser.
Before DefinitelyTyped was converted into a pnpm monorepo, there was the following comment in the index.d.ts
file:
// Type definitions for non-npm package WebExtension Development in FireFox 111.0
// Project: https://developer.mozilla.org/en-US/Add-ons/WebExtensions
// Definitions by: Jasmin Bom <https://github.com/jsmnbom>
// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
// TypeScript Version: 3.4
// Generated using script at github.com/jsmnbom/definitelytyped-firefox-webext-browser
The DefinitelyTyped Firefox WebExt Browser project still needs to work with a few adaptations.