I recall that for Bitcoin, the profit was something like a billionth of a cent per user-hour. So it would be hard to be profitable even if all you're paying for is a little engineer time to deploy it.

I assumed this whole notion of browser mining as a replacement for ads was just a laugh, but maybe not. Are there coins where it can actually make a noticeable amount of money?

That's the reason TPB is using Monero. It was specifically designed to be mined on consumer CPUs with little to no benefit to running on a GPU or ASIC.

That's partially incorrect. While XMR is very resistant to ASICs and not very GPU friendly, a high-end consumer GPU will still mine over 2x as much as a high-end consumer CPU.

edit: of course because the majority of users have integrated gpus, cpu mining will typically be faster.

Bitcoin quickly moved to GPUs and then ASICs, making mining them with CPUs pointless. Some coins are designed to be very difficult to mine with GPUs or ASICs, so a Browser based miner isn't so ridiculous. Wasm also makes it a bit faster.

What sort of return per user-hour would you be looking at with one of those?