Vista Home Premium
BOINC 5.10.18 (but it's been around a while)
Firefox 2.0.0.6
If you click any of the web buttons in the manager, IE is automatically opened despite Firefox being my default browser.
It's also been reported to me via the help desk (I reassured the user he didn't do anything wrong and that the developers were aware of the issue).
I see there's a report of it over at Seti in the last few days.
http://setiathome.berkeley.edu/forum_thread.php?id=41368
For me, Google Talk, Skype and Thunderbird are able to open links in Firefox. A plain old locally written html file also opens in Firefox. Trillian will only open in IE.
I've checked and double checked every place I can think of where Vista might be setting the browser preference.