How high can a Firebird XL go?
Don't know about the Firebird Radio but many of the miniature park flyer type receivers have a limited range. Park flyers are not much fun that high up anyway.
Buzzard, it sounds like you need to try soaring. Or have you. I've had both a 100" and my 2meter Electra/GL up to where you can't make out the tail. Just a vague rectangle for the wing and as it circles around to point straight at you it completely disappears for a second or so.