If you don't care about how hard it is to find these, and just want to get the archives:
Let's cut to the chase: check out Mozilla's default offering to 64 bit Linux users:
Just to be clear, I am not doing any strange trickery here. When I navigate to mozilla.com, the firefox image is what I see. No magical user-agent masking or magic, either:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/13.0.782.107 Safari/535.1
Likewise, if I load mozilla.org/thunderbird I get the Thunderbird image above. This is not some strange user flow bug either, I can get to the download page through the main mozilla.org site, or any other random and/or redirecting links. All signs point to i686.
So, after installing a lot of frivolous i686 libraries on my 64 bit Fedora installation, I was digging around in the Fedora Forums looking for details on some such library, and I stumbled over this. Ok, ubuntu-specific? No, not really. But no real mention of x86_64... wait! There it is, a link to the Mozilla FTP, where 64 bit builds live.
And there they are. Buried in a land where search engines apparently never wander. Well, search engines occasionally wander here, so hopefully I can offer a door to 64 bit Mozilla applications for the Linux masses. Just to clarify, when I started searching for 64 bit versions, I get the blaringly unsupported Windows binaries, forum posts that talk about the fact that there is no official 64 bit build, and a lot of downloads from places like start64.com - which looks a little shady to me.
There are lots of good instructions for installation once you have the archives (including the one linked above at johannes-eva.net that is not actually ubuntu specific). But for some reason, it was overly difficult to actually find the 64 bit archives. Again, here they are:
I just noticed something interesting. The screenshots I provided above, and some of the mentioned URLS were pulled with Chrome 13. I just loaded the Firefox download page in Firefox, and it displays differently, making it even less apparent which release I am downloading (it is still the 32 bit archive).