Call me ignorant, but I thought firmware was , um...free?
It is free but most firmware are also large in file size. So BitTorrent is a good medium of sharing those files.
As for the question, I think mobile torrent trackers never caught on. There were a couple I can remember such as SyncTheBits, Pimpyourmobile, etc but they never gained any poplarity and were soon defunct. Not sure about any new ones though.
nCore. Rich with Android and ios stuff. [Though it's hungarain tracker but the content is english] and, Btw it's General tracker.