it was mentioned here 2 weeks ago but only got 3 votes https://www.reddit.com/r/AndroidTV/comments/7mtlkn/firefox_for_android_tv/
you can sideload it from apkmirror here https://www.apkmirror.com/apk/mozilla/firefox-android-tv/firefox-android-tv-1-0-2-release/firefox-android-tv-1-0-2-android-apk-download/
i have tested it on the mibox 6.0 only. runs smooth. launches with a URL bar and under that is 10 boxes of pre-saved bookmarks/shortcuts. you navigate with a big (too big imo but not huge) blue dot on the screen. it jumps about 1/20th of the screen with each click on the remote dpad and it scrolls the page when you get to page edges. when you need to type something, you move the blue dot to the box you want to fill in and hit the ok button on the remote and the onscreen keyboard pops up. you can complete auto-fills by hitting the 'go' on the far right of the keyboard.
incomplete/missing features: no editing the 10 bookmarks/shortcuts. no saving new bookmarks/shortcuts. no changing the blue dot size (minor in the grand scheme of things). there's a pull-out menu that brings up back/forward/refresh but you can only get to it by mouse not remote (left click on left edge of screen while you're browsing and pull it to the right). the remote's back function works fine but can't do forward or refresh. no tabs for multiple instances.
i haven't tried too many sites because i was mainly interested in testing video. i input a sports stream website i use and the video played perfectly. then i checked a site that uses flash video still and that failed. then i tried another site and i got a please disable adblock message so i guess it has adblock built-in. lastly, i tried amazon's site to play video and that failed. it wouldn't play it in the browser and wanted to open an app to do so. i directed it to the play store and it shows the prime app with no options to do anything. so i directed it to aptoidtv to install from there and it installed fine but that version of prime still doesn't work. it will play a video for a bit then have an error on that video while it plays and eventually punts you out of the video.
TLDR fast, adblock, great with remote. no favorites, no tabs, no flash videos, no amazon prime video.
if you play around with it and find something cool, let me/us know.
Have you tried Puffin TV browser (there's a little write up here )? I wonder how it compares.