Oh, yeah, Commodore Pet, many of us have been enjoying FF x64 4.0b8pre.
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I tend to stay on the latter referenced thread but, certainly, you are free to post any concerns in this thread should you care to do so.
It's been touch and go, at times, with 4.0b8pre x64 (and prior x64 builds) but Mozilla always seems to bring it around giving me hope that x64 will be officially supported shortly after the official release of FF 4.0.