Firefox Extension Auto-update

Supplementary to #190, we'll need to have a website and host an `updates.json` file for the [`update_url` property](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings#update_url) to auto-update the extension, per https://extensionworkshop.com/documentation/manage/updating-your-extension/.