UploadWizard now uses the Firefogg extension (https://firefogg.org/) for on-the-fly transcoding of MP4 files to OGG in the user's browser. But due to changes to how Firefox handles extensions, this is soon to stop working (https://blog.mozilla.org/addons/2016/11/23/add-ons-in-2017/) and it seems already impossible to install the extension.

We should probably stop advertising Firefogg in various error messages shown to users, and remove the code that handles integration with it.

See also T157319: Implement replacement for Firefogg (on-the-fly video transcoding) (but we can do one without the other).