For some reason, when the game starts up, none of the keys seem to work. After the game boots up, click anywhere in the game window, then press Enter - all the keys will work then. Below are the default keys. You can also remap the keys by right clicking on the game title, then choose Options.

Just FYI -- The default keys for FireNES are:

Start: enter
Select: ctrl
A: x
B: z

Use the directional keys to move.