it's not an emulator -- it's a (very realistic) re-implementation of the desktop using standard JS and CSS. Flash is run through Ruffle. Edge opens pages using native iframes.

Essentially the browser split comes from the usual browser split: discrepancies in JS and CSS implementations

This means the developer hasn't tested it on Firefox. Platform compatibility is way better than it used to be but you still occasionally get differences in supported APIs or interpretation of the standard.

I ran it in icefox without an issue, even got a few games of minesweeper in.

The only issue I had was the mobi reader wouldn't work, but that was fine with me.