How to share files with ChatZilla ?

Hello, everybody,

I use Trisquel 6.0, with Firefox and Chatzilla.

I talk with some people that use mibbit on a "closed" cat (we're given the passowrd). I have several questions :

- I can change my nicknanes. It means anyone can change it. It anyone can take my nickname. What to do so that only I can take a nickname (for example : shokin) when (s)he is coming on the chat before I come ?

- I would like to use the possibility to share files. There is one, but it doesn't work often. Why ? do I have to do something before ? depending whether they use Chatzilla or something else ? depending on the browser ? depending what they have done ?

Thanks for your answers.