Firefox preview is really great. Performance is much, much better than the regular Firefox for Android. Now that it supports the most important extensions, there's really no reason not to switch.
Fenix (Firefox Preview) is not currently free software, as it contains proprietary Google libraries like com.google.android.gms and com.google.firebase. F-Droid currently maintain the Google-free fork of Firefox that is available on F-Droid, but they have said they lack the resources to do the same for Fenix as well. Until this is resolved, there is an excellent reason not to switch.
Because a) it's uneccesary and b) the proprietary libraries are in the app, unlike Direct3D where you could theoretically run the program without the proprietary libs (eg in Wine).
Unless you're also okay with Google having root (!) on your phone, it also practically means there's no official package managed source for the binaries. Yes, I can probably grab an apk from some Mozilla domain somewhere (if I can even find it! all their links browseable from mozilla.org simply go to Google Play. The irony of needing to use Google to avoid Google), or I can maybe add an unofficial third-party repo to F-Droid (dangerous! bad!), but if I care at all about sensible concepts like security and regular updates there's no practical way for me to manage it. Even if Mozilla hosted their own official F-Droid repo, that would be better than nothing - but they do not.
Besides, F-Droid is a canary - if you can't get your app into F-Droid, in any capacity, then it's not free software. If you won't do what it takes to get it into F-Droid, you've also staked your colors in the ground - that you don't care about that value system, or about users who have that value system. The message is clear: "Digital serfs only - digital yeomans need not apply".
Some people use LineageOS and others, and don't install platform libraries that are proprietary, so this information is more relevant to them and not so much to you.
There was a post a while ago about Firefox being the browser that calls home the most. I am unsure if Mozilla cared about privacy in a long time really.
Agreed, it is very very good. I haven't used it as my default yet, but regularly switch to it especially when I need to read an article with reader view. It's kind of crazy how a lot of writing on the web is made so unreadable due to ads and poor design choices.
Personally, I've found the browser engine performance to be barely faster (within the margin of error, certainly), but more importantly the browser chrome is now horribly slow. Opening menus and stuff like that seems to take much longer than before.
And as is the nature with such rewrites, it is still missing quite a few little (or sometimes not so little) features that the current Firefox gained over the years...
