I just downloaded mozilla firefox and i keep getting the message?

Question

"The information you have entered is to be sent over an unencrypted connection and could easily be read by a third party. Are you sure you want to continue to send this info?"

Does this mean that it isn't safe to enter passwords and such while I am using mozilla?

Answer 1

If you are on a secure site(url starts with https://) and that website is posting information to non-secure(url starts with http://) then you will get this warning. If you trust the website that you are using then Go ahead with it or else don't.
In other words, if you are using this website for anything important, like bank account etc then don't do it. If it is a simple thing like a survey then go ahead.

Usually, any professionally maintained website(like a bank website) would avoid that kind of posting.

Any new browser would by default ask you this, don't need to be alarmed.