Why downgrade from 9.2 to 9.0 ?Continuing the speculation...
Surely the Mii exploit only needs to be used once. So, you run the Mii exploit, and then use the opportunity to downgrade the N3DS to 9.0/9.2. At that point the browser works again, so no need for the Mii exploit anymore.
Does that make sense?
My AUS / PAL N3DS is on 9.0 and the browser loads up fine.
A possible reason the browser exploit doesn't work on the NEW 3DS, is because the NEW 3DS is using a brand new webkit-based web browser.
The name for the OLD 3DS browser: Spider
The name for the NEW 3DS browser: Skater
Think of it like this, the old 3DS uses google chrome, the new 3DS uses iron(a variant of the chrome browser made to remove defaults of sending back information to google, but sends different information back anyway to the maintainers who knows how they use it.)
I believe smealum stated when Ninjahax got blocked in 9.3 that Nintendo blocked the second stage, which should be After the spider/skater entry point.
But I've never used ninjahax, so I'm not sure if the first point is the ninja game qr code, then the second stage is spider/skater, then etc.
But either way, Ninjahax still worked on 9.2 N3DS consoles, I believe, so shouldn't really matter, maybe gateway just have to add more payloads.
They probably don't want to do that because if Skater's version number ever gets to the same number as Spider's there might be issues with their user-agent based payload selection.
The Ninjahax version warned people not to use the old/new payloads interchangeably, so maybe it wouldn't be ideal to try.