With the version 130, Mozilla introduces a new navigation bar in Firefox for Android. If you don't like it, you can disable it in the "Secret Settings".
I was not a fan when it popped out on my screen. But, after sometime, it's pretty useful and saves a tap on the three dots button.
Hmm, just updated my Nightly. I didn't get that toast, and it's not hiding away for me as I scroll, but I don't hate it. Vertical screen real-estate isn't actually that precious, and though I usually swipe to go back, going forward was a bit of a hassle. It also appears to use the single bar in landscape mode (although currently with an empty black bar above it, but that'll surely go away), where vertical screen real-estate is rarer.