In a new post to the SitePoint PHP blog Troels Knak-Nielsen gives a check up for a project he's worked some on - an interface he came up with to talk, via the dbgp-protocol, to a XUL frontend.

Spectator is a XUL application, which should make it cross platform. I have tinkered a bit with XUL before, but not a full application. [...] So what can spectator do? Mind that this is a first version and I really just meant it as a proof of concept. I think I got a bit further than that, but it probably still has a few bugs. Still, with the current version, you can step through a program, set breakpoints and inspect the stack. Really all you would expect from a debugger.

You can grab the latest version from the subversion repository on the Google Code website.