If your browser signed in to Google, the calendar events will present in the time zone set in your account. If you are not signed in, the times will present in GMT. My guess is that you are signed in to Google when in FF, but not signed in in IE.
I am still wrestling with this issue, since I want everyone (signed in or not) to see the same time with an timezone tag on it. You can apply query strings to the individual event detail pages, but not on the entire feed.
