FireFly sells an external charging device that is compatible with both models, and their battery is compatible with both models; therefore, it is unlikely that using the FireFly1 charger will result in any adverse issues with the FireFly2.
What I was unable to determine, though, is weather the charger that was originally for the FireFly1 charges the battery as rapidly as the new charging cradle that comes with the FireFly2. Charging time with the cradle is purportedly around 45 minutes.
You can charge a Firefly2 battery with a Firefly1 or with a Firefly1 external charger. Firefly1 came with an AC adapter, and thanks to this AC adapter, 80% of the battery will charge within the first 20-25 minutes.
However, the charging time on the Firefly2 varies based on where you plug the USB cable. If you plug it to an iPAD adapter for example, it will charge fully in 45 minutes (and 80% of it will be charge in 20-25 minutes.)