WTF, StumbleUpon?

StumbleUpon is a web service I use for occasional amusement - you register your "interests" and it shows you pseudo-random allegedly-related web pages. There's even a toolbar, which I was silly enough to install for those long evenings when there's nothing on the television and the clients aren't screaming loud enough to get my attention.

Unfortunately it just stopped being funny. I wanted to search my browser history for a specific page, and suddenly I see hundreds of entries, which I am absolutely sure are not a part of my browsing history (I would know if I had seen this page before), listed as "refer.php" whose content reads (typically) "One of our members added your page to the Humor topic on StumbleUpon." Here's a typical example.

So until I get an explanation telling me that either it's a Firefox bug (and we all know how rare those are) or StumbleUpon publicly grovel to get me back they are out, and the toolbar is history, and I won't be going back.