What is nice about these fonts is that they have ligatures, so two characters that come one after the other will be drawn as one. For example != will be rendered as ≠ . Here it is a screenshot:
I prefer Cascadia over Fira because of the rendering of the & symbol, but they are both good and they were designed for programming languages.
I have tested them on NetBeans 11.2 & 11.3 on Windows and it works as long you have a recent JDK, JDK 8 did not work while JDK 13 works for sure. I think this is because the new Font-Layout Engine HarfBuzz that was integrated in Java9 but I have not tested all the versions.