Firestorm allows you to have a display name which is separate from your user name.
NOTE: Display names can only be changed once a week; once set, you have to wait a full seven days before you can change it again. However, a display name may be deleted sooner than that.
To change your display name, you need to have them enabled for you. So first check that you do, by opening Preferences (Ctrl-P), then going to the General tab. Make sure that View Display Names is enabled, then click OK.
Now bring up your profile by right clicking yourself and selecting Profile from the menu.