[Greasemonkey] can't install any scripts with GM 0.5.1

[Joseph Grossberg]

Whenever I try to install any user scripts, I get one of two errors:

1) "Nothing happens", and the JS console shows:

Error: uncaught exception: [Exception... "Component returned failure code: 
0x804b000a [nsIIOService.newURI]"  nsresult: "0x804b000a (<unknown>)"  
location: "JS frame :: chrome://greasemonkey/content/scriptdownloader.js :: 
ScriptDownloader :: line 49"  data: no]

2) Other times, I'll get the "Install User Script" dialog box, which appears 
to have no data in it (e.g. a script name, included pages, excluded pages). 
I click on "Add" to add included pages, and when I hit "OK", those are added 
to the list in the dialog box. However, when I click the "OK" button for the 
box itself, nothing happens -- no JS error, the box does not close, etc. 
Only "Cancel" works.

Other info:

This only happened after I upgraded to the most recent version. I had none 
of these problems while using the 0.3.5 patch or any of its predecessors.

I tried the uninstall-close-launch-install-close-launch dance too, and that 
did not solve the problem.

FWIW, I'm running Firefox 1.0.6 on Windows XP SP 2.

Does any of this sound familiar? Can someone give me pointers as to what's 
going on?

Thanks in advance,
Joe