When I'm reading forums I use the mouse scroll wheel a lot, and sometimes I accidentally click the middle button at the same time. When I do this, FF pops up a message saying that it's unable to connect to the selected URL. All perfectly normal.
But this morning, when I was quickly scanning through some messages on TBB, I suddenly got transferred to a site offering male enhancement products. (That's my story and I'm sticking to it.) For a moment I wondered if I'd encountered the first successful Linux trojan or something like that, but then I remembered the scroll wheel issue.
So I paged back to TBB and looked through all the messages to see if I could find the dodgy link, but I couldn't. There were a few links in people's sigs, but they were innocuous. I was unable to repeat the experience. So now I'm just a little bit puzzled as to what happened.