TL;DR

Firefox Android is getting an update (v68.x -> v79.0) that is a complete rewrite of the browser and a downgrade in many ways until it is feature-complete. Don't update, or use Fennec F-Droid or Bromite until it is mature.

Background

So Firefox has been on Android for ages, and is a really good browser. The only one that is still well-supported and has a full extension ecosystem. Just being not Google is a great reason to use it but even then, it's actually a good browser. I would go so far as to say that after some privacy hardening, it's the only browser anyone should use on Android. (I can put up a post about that later, but privacytools.io is a decent starting point). The current version (68.x) is a project called Fennec internally by Mozilla.

Over the last couple of years or so, they decided that the Fennec codebase needed an overhaul so they undertook what is essentially a full rewrite of the UI as well as the web engine it runs on. This project came to be known as Fenix. The promise was that it would be a prettier, slicker, faster, feature-complete browser that would offer everything Fennec does with several improvements on top. Fenix was released first as a private alpha, then as Firefox Preview, and then merged with the Nightly and Beta channels of Firefox.

It was assumed that it would be maintained as a separate project till they achieved at least some semblance of feature parity with the current Fennec version. But it seems like they have already started rolling out Fenix to the stable branch, effectively replacing Fennec on the Play Store as Firefox v79.0 This started a couple of days ago and they are planning to replace Fennec entirely by mid/end August. You can check the status here.

The Problem

Fenix is actually pretty slick and nice to use, but it has several issues that show it is nowhere near complete.

1. At present, Fenix only runs 9 extensions and has no access to the Mozilla Add-Ons Store from where extensions are downloaded. This is a serious downgrade.
2. The stable version of Fenix that has rolled out to users disables the 'about:config' menu that is used for a lot of under-the-hood changes in Firefox, from privacy hardening to UX tweaks.
3. Even among existing add-ons, there are bugs. For example, I use uBlock Origin in medium mode + JavaScript disabled by default. This means occasionally I have to change some rules at a granular level to get a site to render properly. It works fine on Fennec, but on Fenix sometimes uBO doesn't recognize what tab it is working on/doesn't correctly show the options to change rules and reload the page. I actually liked Fenix and was using it before I had to bounce off because of this.
4. You're likely to run into more issues yourself lol, considering it is an unfinished product.

The Solution

It's worth noting that Fenix is not a bad browser in the least. I think when it is finished it will be great, but they have no right to foist it into the stable channel in its present state.

Your options at the moment to not auto-update are:

1. Switch off auto-updates for Firefox on Play Store
2. Uninstall Firefox from Play Store, remove it from your library (to prevent future upgrades) and install either Fennec F-Droid which is basically Firefox with their branding and telemetry stripped away, or the latest Firefox APK.
3. If you don't want to use Firefox anymore but still want a decent browser that has ad-blocking and some privacy hardening, use Bromite. It is a fork of Chrome that is functionally the same but has superior security and privacy enhancements. I do not recommend Kiwi Browser because it isn't updated anymore and hasn't been in a year.

Point 2 and 3, in my opinion, only serve as stopgaps until Fenix is a full-featured mature browser itself and you can return to it. Firefox is still a great browser that everyone should use, and actively promote over Chrome and its derivatives. This post isn't meant to hit out at Fenix devs, they are doing good work and bugs are expected with huge projects like this. I believe in a few updates they will definitely reach feature parity with Fennec. That said, I do object to decision-makers who decided this was the right time to push it to the stable channel.

Live long and prosper, my good libs.