These are instructions on how to find the firmware and baseband version (modem firmware) that are currently running on your iPhone.

Click on "Settings" on the Springboard of your iPhone.

Click on "General" from the "Settings" menu.

Click on "About" on the "General" menu.

If you now scroll down on the "About" screen, you can find your firmware version under the "Version" label.

If you scroll even further down the page, you will find your baseband version under the "Modem Firmware" label.