Rated 4 out of 5 stars
I tested this extension with Firefox 3 beta 4 on Ubuntu Linux 7.10
Using the Saxon B xquery implementation worked without any problems and was quite useful to query information from large documents. I was unable to download the Berkely DB to test that because oracle's site just hung forever when trying to download it.
I noticed no errors or warning in the error console while using the extension. All in all, a useful extension if you want to query information from documents on the web and are familiar with XQuery.