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.
