Arrow - you've said it for me!
I've used it on all those OSs (and on various Linux setups), and on much older pcs (this one's an 8 yr old laptop, 64mb, 350mhz, running W2k and 98se). On older equipment the difference is even more marked - KM on this machine opens in about 5secs on a good day, whereas FF never manages less than 15secs! Also, there are still one or two niggling little bugs in FF that have never been fixed, but you'd have to be a 'keyboard only' freak to notice them
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I'm no developer, but I believe it's do with KM - mainly - using native windows system files, and using macros instead of extensions, as in FF. It's just mean, sleek and unpretentious, and I've used it since version 0.8 almost exclusively. This latest build is the meanest, fastest, safest browser out, and I wouldn't use any other unless I had to.
Also, this forum is one of the friendliest in cyberspace, as I'm sure everyone would agree!