Right click the Firefox shortcut and click Properties.
On the Shortcut tab, set the "Run" line to "Normal window" and OK.
Try Firefox.
If it still is missing the brand bar (where the min, restore, and close buttons are), open the Properties again and set the "Run" Line to "Minimized" and OK.
Try Firefox. It should open as a button on the Taskbar. Double click the button to bring it up.
If the brand bar is there, set the shortcut to "Normal Window" again and OK.
If that doesn't reset things, you will likely have to do a clean reinstall.
Make a COPY of your Profile folder to Import your bookmarks and settings from.
T.
Edit: This assumes it's only Firefox with the problem and no other windows are affected.