Summary: Firefox Browser is the stable consumer product. It’s about to be replaced with Firefox Preview, a major new release you may want to switch to early. There’s also a collection of betas and nightly builds.
One thing that’s nice about Android is that Firefox works nicely on it; there’s no iOS requirement that all browsers really just be reskinned versions of the vendor’s browser component. I’ve set up Firefox to be my default browser and that setting mostly sticks other than Google launching Chrome occasionally on me. I think Firefox is even used as an embedded browser in other things, but I’ve never been sure.
Anyway, the confusing thing is there’s so many things called Firefox on the Google Play store. Here’s a rundown of them all.
Firefox Browser (org.mozilla.firefox): this is the one you probably want, the mainline consumer app. This is currently version 68.8.0. This is the old Firefox that’s been around for awhile; this is not the new version announced in January 2020.
Firefox Preview (org.mozilla.fenix): this is the new hotness. Version 4.3.0 for me right now, but also says “GV: 76.0”. It’s a major update involving a port to GeckoView. See also these details of new features. Dark mode, more privacy, an option to put the toolbar on the bottom of the screen (near your thumb), a Private Browsing button right on the tab selector. Main change though is a full new rendering engine. Supports fewer addons, but does have uBlock Origin. This new stuff is slated to be released to the public as the Firefox Browser pretty soon.
Firefox for Android Beta (org.mozilla.firefox_beta): this looks like a slightly older version of Firefox Preview. Currently reports version 75.0.0-beta.6, also “GV: 75.0”. I would guess ordinarily this would be the cutting edge beta version of Firefox but right now during the transition to the new hotness, Firefox Preview is getting updates faster. Not positive.
Firefox Nightly for Developers (org.mozilla.fennec_aurora)
Firefox Preview Nightly for Developers (org.mozilla.fenix.nightly): I didn’t test these. I assume they’re just nightly builds of the two main development lines. FWIW the Preview Nightly currently reports “GV: 78.0a1” so it looks fairly far ahead of the main Firefox preview
Firefox Focus (org.mozilla.focus): a privacy-oriented experimental browser. Version 8.2.0 (75.0) right now. I think this was most relevant a couple of years ago and has taken a backseat to what is now Firefox Preview, which includes some of the same privacy features.
There’s also a bunch of stuff using the Firefox brand name that isn’t a browser at all. Firefox Private Network VPN (org.mozilla.firefox.vpn): is Mozilla’s new project selling VPN service to users. Firefox Lockwise (mozilla.lockwise) gives access to the Firefox password store. I think they’re trying to become a password agent like 1Password? Firefox Send (org.mozilla.firefoxsend) is a tool for sharing files. Last update was March 2019, not sure if the project is abandoned. Notes by Firefox is yet another text memo product. Firefox ScreenshotGo organizes screenshots with OCR and text search. And finally Mozilla Stumbler is a wardriving tool for building out Mozilla Location Service data. Last release Feb 2019.