i switched from firefox to waterfox on my phone and have yet to spot any difference.
i heard some websites might have issues (e.g. web versions of office software) and some settings are missing but in my own everyday use i didn't run into any of those yet.
Librewolf is definitely better if you are a hardcore privacy nerd. I used it but it broke some sites due to the privacy settings. Waterfox is better than Firefox privacy-wise. Waterfox is in the sweet spot for me. Waterfox has an app but I do not like it because it was very slow compared to the Firefox app.
I use Waterfox as desktop browser and Firefox as mobile browser.