Oh my god. That website has javascript that adds and removes a pipe "|" to the end of the page title, and it looks like a blinking caret for text input.
Looks like it only alternates between the two states (flashing and not flashing), so if the page is opened as a new tab (i.e. not focused to begin with) then the flashing occurs when the tab is not focused.
If I place it as an application tab in Firefox (16), it will highlight the tab because it assumes the content has changed... it doesn't happen always, though.
My OCD is going insane.