Add "update_url" to the GitHub uploaded Firefox extension manifest

[pointydev]

After a brief conversation about a segment and it's timing in the Discord, I noticed that I was on version 5.5.9 of the Firefox extension when 5.11.5 was available, and that I hadn't been receiving any automatic updates for the extension. I had previously installed the beta version to fix an issue at some point, so I wasn't receiving updates and didn't realise.

I believe you could add an update_url field to the extension manifest for Firefox builds hosted on GitHub, meaning users of the beta extension wouldn't need to update manually, making testing easier. Mozilla provides documentation on how to do this here, with the only changes needing to be made being the addition of the field, along with an update manifest being hosted that contains the location of the latest update (see how uBO does it here as an example).

Thanks,
Elliott