It's weird because Firefox is has a lot of fairly basic UX problems, such as imposing mobile conventions on a native desktop application instead of following native conventions.

Besides the missing top menu bar already mentioned, desktop applications don't have hamburger buttons and material design-aesthetics, looks really out of place. If you open a sub-menu in that menu, the entire menu is replaced with the submenu, that's also not how desktop menus work.

I hate the sub-menu thing as well. Because of the way it works currently, a sub-menu doesn't auto-expand when you hover over the parent menu, you have to explicitly click on it, and same thing when you have to go back. I have to access the "Reopen all tabs" function from time to time, and I can never remember where that menu item is, so I have to click around to find it, and have to explicitly click to go back to the previous menu when I make a wrong guess.

I miss being able to put a traditional menu bar next to the URL bar (and with the "Help" menu removed). I've trimmed the excessive whitespace with custom CSS, but it still wastes more vertical pixels than it used to do several years ago. There's no reason the URL bar needs to stretch all the way across the screen. Human-readable URLs are not that long, so the space could be put to better use.