Hi,

I'm having my first real go at editing my own Gtk theme and I'm finding myself a bit lost. I'd like to find a widget browser to show me the classes of all the widgets in a window.

Does such a thing exist?

It would be super useful for things like Firefox where the widgets aren't what you would expect.

I am afraid Firefox doesn't use properly GTK2 as you think. You should go to mozilla site and try their recommendations for userChrome.css and userContent.css
Chrome is what controls how XUL (Mozilla/Firefox engine) looks

However for GTK2 theme reference and exploring widgets you should use a default eXperience engine theme (having the engine installed of course)
It offers debugging from a terminal, which permits knowing the exact name of a widget as you click.
For more information on this please visit the group GTK themes in: http://gnome-look.org
I have posted an extensive help on this topic there.

The official site of the engine is http://art.gnome.org/