The mobileread wiki has a page that has been listing most of the Kindle firmware versions released by Amazon.
Unfortunately it hasn't been updated for quite some time now.
I don't want to keep up with the wiki, so I will be listing on my own the versions I have found on the Internet.
Unsure if this is the right order. Some of these versions weren't necessarily released through the Amazon website.
You can still download these versions with the right URL.
Learn what Kindle model you currently have. There is a lot of websites that explain this.
Download that firmware version. You can use one of the following sources: