In theory you would be able to do it for certain tablets - according to this article:
http://www.cnet.com/uk/news/microsoft-cooks-up-way-to-run-windows-10-on-android-devices/
But this is for a Chinese tablet called the "Xiaomi Mi4":
"In an update to TechCrunch's original story, Microsoft confirmed that Xiaomi Mi4 owners will be able to "flash" Windows 10 on their devices. "This partnership will allow Xiaomi and Microsoft to get direct user feedback and continue to improve the experience for China. Microsoft is thrilled to see Xiaomi embracing Windows 10," Microsoft said in a statement to TechCrunch."
"So far, the program is limited to China and just to Xiaomi. So this doesn't necessarily mean Microsoft will expand its Windows 10 transformation technology to other companies or countries. However, Microsoft reportedly does have plans to make its custom Windows 10 ROMs available to more devices in time, according to TechCrunch."
The Kindle Fire won't be powerful enough to run it properly, and the ROM isn't offically available for it.
If you want Windows 10 on a tablet, you could consider going for the Surface Pro 3:
http://www.microsoftstore.com/store/msuk/en_GB/pdp/productID.300212300
I would suggest going for the i5 8GB combo at least though, as 4GB RAM will soon seems like too little.