I don't see that as "forcing" anyone to do anything.
By not providing the latest Chrome version, XP/Vista users are effectively being "forced" to use Firefox, since using a outdated web browser actually isn't a option these days.
Firefox is not and never has been an option here.
FF is way more secure than IE.
The only question is Chrome vs. FF, with the former out of the picture, the latter is the only thing left.