It does have a bottom toolbar and was configured like that by default before. They must have changed the default some time ago.

System UI is just standard Google Pixel UI and it reacts to apps signaling to it what color theme it’s in. Firefox just isn’t signaling it correctly. It also should be reacting normally to dark mode toggle but it doesn’t because the main Fx toolbar doesn’t change to light mode in the mornings. No other app has this issue.