In practice, how secure is a tenured position in the US?
I haven't seen any statistics on how many tenure professors have been fired, but most articles on the topic treat tenure as though it's a lifetime position (e.g., this Science article, "Tenure and the Future of the University"). Anecdotally, you will likely never meet someone who knows someone else who was fired from a tenure position; it simply doesn't happen.

Note, however, that the number of tenure track positions made available over the past decade been trending downward fairly significantly (see the same article, and simply do a google search on the topic to see more).