I use the extension and Firefox 1.5, too, but find it to be more likely GreatNews bug than the browser/extension. To reproduce the problem:
1. Start -> Run "feed://test.test.com/test.xml"
2. Untick "Retrieve Channel Properties From Internet" and click Next
3. Click Cancel
4. Repeat from step 1, the error will appear
One more thing, GreatNews stores the feed URL with feed:// protocol. Since feed:// is actually just a pseudo-protocol of http://
, I propose that GreatNews rename/replace the "feed://" with "http://". When the channels are exported to OPML and imported to other readers (in other machines), I reckon it will have less problem with "http://" than "feed://"
PS: just a reminder, if you decide to do this, you may want to 'upgrade' the DB to do one-time "feed:// to http://" replacement.