As the draw on a Fire is higher than most cell phones, I would be careful. What is the rating for the charger? My experience has been that not all car chargers are equal even if the connector is the same.
If you are using the Fire while it is charging, you may find that a 500mA charger is not enough to keep up, ie the battery will slowly discharge or stay at the same charge level.
High current chargers will charge faster, but only to a certain extent. A standard e-Ink Kindle's maximum current draw is about 800mA. For a Fire, if you want to use it at the same time as charging it, it can draw up to 1.3A or more. A higher charger current than the standard charger (850mA for e-Ink, 1.8A for Fire) will not charge any faster than standard.
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