Although Xfce 1.0 appeared slightly earlier, its initial releases were proprietary, making what was originally known as the Kool Desktop Environment one of the earliest entirely FOSS desktop environments for Linux. Version 1.0 was released in July 1998.

What a weird opening.

Also, both projects were started late 1996. A lot of people were already running KDE alphas/betas before the 1.0 release, because as a desktop it was vastly better than any of the window managers at the time.

I remember installing beta 1 in 1997 was a big relief for me. It felt like the last barrier was broken for Linux taking over the desktop, now that it finally had a desktop that could compete with Windows 95. Of course, I was terribly naive, but I blame it on only being 15yo.

I don't recall Xfce being used much at all. It become more popular until 5-10 years later because people were complaining that KDE/GNOME became too heavy and they were looking for more lightweight alternatives.