Knowing the species would be important since the name "firebelly newt" is used for various species from up to four different genera, with different biologies and needs. Mind you, most of them will do very well in a heavily planted tank.
Also, before you buy any "firebelly newts" please know that all of the newts available at pet shops are wild-caught and imported in terrible conditions. This takes a huge toll of lifes and suffering every year. Also, as a costumer you'd be acquiring an animal which is likely to do badly, as they frequently develop infections, refuse to eat, etc...
If you want newts, first make sure to get properly informed, which means reading until your eyes bleed. Also make sure to have everything ready way before hand, tank cycled, food cultures going, etc. Only then should you consider which species is most suitable to you and start looking for breeders who can provide you with healthy, captive-bred animals.