You may be able to download and side load install the Android Firefox apk file. https://archive.mozilla.org/pub/fenix/releases/145.0/android/ Not sure what kindle fire you have so you may have to look up the cpu it has. If it is say the Amazon Fire HD 10 then try the fenix-145.0.multi.android-arm64-v8a.apk Mozilla actually did have Android Firefox on the Amazon app store in the early days of Android Firefox however it was found that Amazon modified the file and it caused problems with the updates and Amazon had not been willing to fix it since.