installation returns status of -210 to install callback

I'm trying to get XPInstall to work (on Firefox 1.0.6/Windows) but am
blocked at
the moment. InstallTrigger.install is getting called via a function invoked
"onLoad" of an html page. In my InstallTrigger callback, I can see that the
return code from the installation is -210, which seems to be USER_CANCELLED,
although there's no dialog asking me to cancel installation.

In my install.js inside the .xpi, I've got at least one alert that should
pop unconditionally but it never does so I figure install.js is never even
getting loaded.

I've tried this in Firefox 1.0.6 and Netscape 8 and get the same result.
Netscape 8 at least has an install.log file (why doesn't Firefox?) but I
don't see anything in there related to my install.

Does anyone know what I'm doing wrong or have any suggestions?

Thanks very much,

Larry