First, you'll want to make sure all of your personal and account information has been erased from the system. And that's what this blog brings to the table: instructions on how reset your Kindle Fire back to factory settings.
Before we get that far, though, it's probably a good idea to make sure you've backed up any absolutely crucial files that have made their way onto the device. Once it's erased, it's gone for good and unless it's an app, movie, or an MP3 purchased through Amazon, getting side-loaded apps back onto the Fire won't be as easy as simply redownloading them from Amazon's servers.
OK, with the PSA out of the way, let's get started.
Check your battery life
The Kindle Fire won't allow you to perform a factory reset until its battery is at least 40 percent charged. You can confirm your Fire's battery life by going into settings. To do this, first, tap the cog symbol in the upper right corner and then tap More.... Scroll down and tap Device.
There you should see your Fire's remaining battery life as the second entry. If it's not at least at 40 percent, plug the Fire in and charge it until it is. It's probably a good idea to go a couple percentage points over, just to make sure.
Let the erasure begin!
Now, following the instructions above, go back to Device settings, scroll down and tap Reset to Factory Defaults. Then tap Erase everything.
This will deregister your Amazon account from the Kindle Fire and the device will reboot. Rebooting takes a couple of minutes and once back up, you'll find no trace of your personal information, passwords, browsing history, etc.
You can now rest assured that whether you're planning to regift it, sell it, or throw it in the trash (don't do that one; at least get it recycled), no one will have access to any of your personal Kindle Fire history.