AFAIK you have to select DASH as your preferred video quality (or append &quality=dash) for other resolutions to work. It's not enabled by default because the server has to proxy the video (CORS):

https://github.com/iv-org/invidious/issues/177#issuecomment-...

You need to enable the DASH format in the settings if it isn't already, I think.

You gave me doubt, I just checked and 1080p is available on Firefox in my private Invidious instance.