window.showSaveFilePicker and window.showOpenFilePicker not supported on firefox

[axellse]

Open notebook and create notebook dosen't work on firefox since they use window.showSaveFilePicker and window.showOpenFilePicker, see the compatability table. This could be fixed by using an input element with type file, or just adding a notice that firefox isn't supported.

I'd also like to say that this is likely the best project i've seen on high seas so far, so well designed, everything looks very pretty and the command bar is such a nice addition!