If it's from an add-on, disable the add-on.
If it's a toolbar (like the address bar, the navigation bar, or the bookmarks toolbar) right-click in an empty part of the toolbar and uncheck the box for that toolbar.
If you click customize you can move things around. I stuff everything up into the top bar, and use All-in-one sidebar and tree-style tab to maximize vertical screen real-estate.

Also, this site has broken CSS. It forces black text, but I'm using a dark theme and should have light/white text in the edit-boxes. I end up editing all my comments in EMACS.