Is it possible to change this gradient to something else (preferably a solid color or the default gradual horizontal fade)?
Using Firefox 4.0 on Windows-XP-x86-Pro-SP3.
Sadly, Mozilla focused only on Windows 7 for their slick Firefox 4 UI. So, people get an additional title bar in older Windows'.
In a nutshell, no it's not possible. =/
I would think it is possible but you would be changing the color of all windows title bars - not just for Firefox. To change title bar colors you right click on the desktop and select properties. You can change the appearance of windows somewhere in there.