I am aware of that. But you shouldn't need to use a third-party extension just to watch video at the correct aspect ratio. It should just work on ultrawides, without any additional fiddling.
Im sorry but are you on a console or a pc? cause it sounds to me like you are expecting the experience of a console where everything is simplistic plug and play. Wouldn't personally blame the ultrawide format for netflix's encoding choices. You either get black bars on a 16:9 but perfect full picture in 21:9 or the other way around but there will always be someone with black bars. I would rather have a 21:9 with black bars from time to time than 16:9 with black bars top and bottom but thats just me and my preference.