There are a couple of XUL widgets which aren't finished yet, so they aren't documented on XULPlanet. One such widget is the <spinbuttons> tag, which is a textbox with two arrow buttons for adjusting the value. While there is some code for it, it is missing both images and code to do something when the arrow buttons are pressed. That said, finishing it is something that anyone with a bit of XUL knowledge can do, perhaps with a bit of help. (meaning, no C++ knowledge is needed.) If you'd like to help, let me know.
The other widget is the <slider> tag which allows one to select a value by sliding a thumb, much like a scrollbar. There is such a tag in XUL, however, it is really just a piece of a scrollbar, rather than a completed widget. The first step is getting it to look like a slider. I spent a bit of time with this, both with the slider code and the underlying GTK theme code. Here's an image, and I'd say it looks pretty good, and it works too.
I'll probably try a Windows version soon. The other problem is that the slider needs a few extra features to make it more useful as a slider, rather than a scrollbar with a different appearance. Most importantly is to display the current value in a label nearby. Not too hard, but the shared slider/scrollbar code assumes that the valid range always starts at 0 and ends at the value of the maxpos attribute. A minpos attribute could be added easily enough, but there are also cases where the slider might be used to choose a percentage, a decimal, or some other fixed set of strings.
Another common slider feature supported by other toolkits is showing tick marks along one edge. Unfortunately, GTK doesn't seem to support tick marks in its slider, so I don't think it's possible to make ticks theme-indepependant.