I think it looks and feels much better than before.
1) by default tabs are now white on a white background, with the active tab only being a slightly darker grey.
2) Everything is much more crammed together (tabs and bookmarks toolbar especially) which makes sense on a small laptop monitor, but on my desktop is just annoying.
3) Dialog boxes to confirm closing of multiple tabs have white text on grey background. (?!)
4) Folder icons in bookmarks toolbar now have this retro black and grey design.
5) Tabs are at the top of the window with no gap, even when not maximized, making window dragging more difficult.
In many ways it feels like a throwback to much older designs.
If you've migrated from a previous version of Firefox and customized the browser at all, you may not be getting the default settings, though-- right-click the toolbar or Bookmarks Bar, and take a look through the options at the bottom of the page. Themes will give you an option between the default, light, and dark themes (it sounds like you're stuck on the light one); density adjusts spacing in the toolbar area. And you can check "Drag Space" to add space above the tab bar for dragging, or check "Title Bar" to turn the system title bar back on.
And thank you for that drag space tip. Definitely part of what I was looking for. I just assumed it toggled the ability to drag whitespace around while customizing!