So i have a spare Kindle Fire power adapter: it's quoted as 5V @ 1.8A. I haven't tried as I don't really want to risk blowing it up!
Yes. The Raspberry Pi model B only requires around 500mA of current, and it's maximum consumption is around 800mA. The Pi will only take as much current from the power supply as it needs. The only problem that could occur is that if your power supply is faulty and does not give the voltage it should.
also, is this question a duplicate of Can I feed the device with a current rating that's higher than what's recommended??
Yes. So long as the charger is 5V and AT LEAST 800ma, you should fine. If there is question as to the operation of any 5V supply, test TP1 and TP2 as noted here: http://elinux.org/R-Pi_Troubleshooting#Troubleshooting_power_problems