Enable support for Firefox Mobile

[felipemarinho97]

I have tested your extension in Firefox for Android and it's working fine!
You just need to add browser_specific_settings.gecko_android property in the manifest.json.

[felipemarinho97]

For anyone who wants to use this on Firefox Mobile, I've made a fork: https://github.com/felipemarinho97/Overseerr-Assistant-Reborn