I honestly didn't notice any difference in performance when Firefox Android Beta was switched to the new codebase a couple of weeks ago. Not saying it's not there, just that I couldn't tell.
I have trouble getting used to the new UX.
I keep hitting the address bar to open a site from my top sites in the current tab. That used to work, now it just opens the search interface. Instead you have to hit the -- much smaller -- tab overview button to open a top site in a new tab. Which means I end up having to manually close old tabs all the time now, something I don't remember doing. (To be fair, Chrome works the same way.)
I don't know how I'm supposed to use the new Collections feature. It's extremely prominent in the UI, I assume everybody else must be finding it super useful?!
They broke the offline reading list? You used to be able to bookmark a page in Reader Mode and have it available even when there is no connection (subway, airplanes). Instead of promoting this really neat (and given that Reader Mode is still a thing, cheap) feature, I think they just got rid of it. I had like a dozen of long articles in that list I still wanted to read (I think the bookmarks are still there). Hopefully they'll restore it before it transitions from Beta to Release, but who knows.
