I'm pretty sure this JavaScript mining executes the Cryptonight algorithm, which is used for Monero mining. I tested it with a 4-th gen Intel Core i5 and it achieves about
60 H/s, which leads to roughly earning 1 satoshi per minute in your FreeBitco account, so unless you have very cheap electricity you will mine at a loss.
This was to be expected from a JavaScript implementation. A native CPU miner is much faster, for example the same processor achieves
130 H/s (i.e. 2 times more) with the
Nicehash miner and currently they pay roughly 2 times more for mining Monero, so currently you will get
4 times more bitcoins mining Monero with
Nicehash using your CPU. If you have a modern graphics card you can earn even more. That's why currently it's better to use
Nicehash for earning Bitcoins and then if you want you can easily deposit them to your FreeBitco.in account for the interest.