A few suggestions

[CHJ85]

Please add the fullscreen option. In Chromium browsers, the flag is "-start-fullscreen". Not sure about Firefox. This is useful for sites that such as Netflix, Hulu etc where you want it to be full screen.
And there's also a bug I wish you'd fix. In isolated browser mode, the browser gives you this message asking if you wish to make it your default browser and send feedback. I believe this setting is in the Local State file. Not sure how to do it in Firefox though.

[PizzaLovingNerd]

Please add the fullscreen option. In Chromium browsers, the flag is "-start-fullscreen". Not sure about Firefox. This is useful for sites that such as Netflix, Hulu etc where you want it to be full screen.

Does firefox have a start-fullscreen function?

And there's also a bug I wish you'd fix. In isolated browser mode, the browser gives you this message asking if you wish to make it your default browser and send feedback. I believe this setting is in the Local State file. Not sure how to do it in Firefox though.

What browser is this happening in?

[CHJ85]

What browser is this happening in?
Chromium browsers.
Does firefox have a start-fullscreen function?
Yes. They call it Kiosk mode. Command is -kiosk.

[PizzaLovingNerd]

The chromium browsers I don't think we could fix because these are bugs with the chromium browsers upstream. However I'll look into adding a kiosk mode.