The sideloaded Firefox APK for Fire TV works fine on my MiBox, and there's the added benefit that it comes with its own mouse cursor (circle) that responds to all directional clicks on the MiBox remote.
The downside, at least of the version I have, is no bookmarks functionality, and no ability to save quick favorite sites as icons on the home page, and basically no other customization. Also, if you end up using it to go to YouTube, darned if I can figure out how to get the browser out of YouTube once there, short of going into the app settings and clearing cache and data. The back and home buttons don't seem to get you out of YT once there.
Perhaps because the whole purpose of Amazon releasing a version of Firefox for the Fire TVs in the first place was to provide an alternative for accessing YouTube once Google cut off access via the normal YouTube app on Fire TVs.
With Chrome, depending on the website, the directional clicker can scroll down and select some page elements. But others it simply can't highlight and select, so I end up having to use an attached PC mouse plugged into the USB port on my MiBox to navigate a lot of page content.