To play the note C2 you hit the /? key on the keyboard, but this is bound to "quick search" in firefox, with no way to disable it without an add on. I assume it just needs a preventDefault somewhere

alternatively you could rearrange the keys so it's not needed, I'd prefer if it used the FL Studio layout, since it puts C for both octaves on the left side of the keyboard (Z and Q), which is a bit more intuitive.