Messed around with an older Kindle Fire tablet this weekend, and stumbled-discovered you can side-load #fdroid through the Silk Browser. From there, you can install any of #deltachat, #ironfox, #orbot, #comaps,... even #supertuxcart

Installing apps through #AuroraStore is hit-or-miss, where the 'misses' are very informative: #firefox & #fennec must have hooks to Google Play Services, because they immediately crash upon opening in FireOS.