I hate new address bar with new tab button in Firefox Nightly for Android.
It reduces vertical space and so annoying!
Here's how to disable the new "Navigation Toolbar" and revert old address bar.

Enable "Debug menu" and change "Secret Settings"

1. Tap on three-dot menu in the toolbar and open "Settings".
2. Select "About Firefox Nightly".
3. Tap the Firefox logo until you see the "Debug menu enabled".
4. Go back to Settings.
5. Select "Secret Settings".
6. Turn off "Enable Navigation Toolbar".

That's all! Bye-bye Navigation Toolbar!