What does being firm and nice at the same time entail?
There are things which are important and things which are not important. We should know the key things in a matter ( or in life), which we need go a certain way for our needs i.e. if we don't express ourselves in these matters then we might still be nice but the niceness may lose it's genuineness because it may not be sincere anymore-since we are not expressing what we are feeling ( i.e. we might be building feelings inside of us--and like a pressure cooker being opened all of a sudden----we may damage ourselves and others when we finally come out with our feelings-explicitly or implicitly).
Once we know what we want then it is advisable to ponder our intentions, goals, means and how we take those means to get to our goal--making dua throughout.
Being firm and nice also entials picking our battles carefully. There are things we may be able to let go and other things which we may not be able to let go of. If we get stuck up on everything (even if you sound nice doing it), we will lose respect of others.
Another key is that ( in matters which really matter to us), we express our firmness in the right way at the right place at the right time. It may not be best to vent out a friend when there are 5 others watching even if what your saying is right and you are saying it very nicely.
Being firm means being firm in achiveing our goal. Sometimes we become firm in "how" we reach our goal as well i.e. this is how I want to do it even if it alienates everyone ( when there may be a softer approach to do things available).
There are matters where we might not know how to go about doing things in a nice, firm effective way. At that point it may be advisable to seek advice ( from the right people)
As for being nice. It entails smiling and asking someone for something ( scholars say even for small things we should ask Allah first, since it is Allah who gives everything). It entails not gettiong angry if the other person responds angrily or is rude ( the sunnah is to return the bad with good). We should forgive and forget and move on from past issues with others as soon as we can. Also when we ask someone something, we should be prepared to hear an answer different then what we would like to hear. It may be advisable to be ready for a dilogue and not a monologue. Sometimes we might be firm on something but might not understand the issue. Also it is less offensive if we take someone through a thinking process when we are trying to convince them ( i.e. ask directive questions which make them think).
And what matters are ones which we should be most firm on? Things which are of next worldly benefit. How do we learn how to be firm and nice? Learn and read about the Sunnah.
We ask Allah for Tawfiq and Taiseer in all our affiars. Ameen.