Right, but existing Firefox for Android users aren't going to be switched over to Fenix until it does.

Not to be mean, but that statement demonstrates a frankly shocking level of cluelessness. Extensions (mainly adblockers) are the critical feature that distinguishes FF mobile, and are the only reason I have ever been able to convince anyone to switch. I would almost consider adblockers functionality more critical than Javascript support, they should certainly be part of a minimum viable product. I know it's a preview version, but why would anyone in their right mind run this?

Possibly someone in their right mind would run it because they read TFA where it says "primarily aimed at developers and early adopters who want to help us improve Firefox on Android".

I would say that adblock/extension support is a pretty big deal breaker for "developers and early adopters". How are you going to get them to use a crippled browser for any significant period of time?

You're not supposed to use it for an extended period of time, that's why it has "preview" in its name.

I'll be happy to use their other browsers (mainly, Firefox Focus, my default browser) until this one gets more polished, but the moment this one becomes a bit more stable and adds a few more features (adblock support is certainly on top of that list) will be the moment when I not just switch over, but delete every other browser I have installed on my phone. I can't believe a browser (any browser) makes me feel excited in 2019, but this one does.