We have 64-bit builds for Firefox, but we don't promote them. I believe that is because they are not QA'd to the same level (but I'm not sure). In any case, if someone is *already* running a 64-bit build, I think it makes sense to offer them a 64-bit build when they visit http://www.mozilla.org/firefox/new/ A user already running a 64-bit browser knows the relevant issues, since they got it in the first place, and they probably want a 64-bit build or if not are capable of getting a 32-bit build.
http://nightly.mozilla.org/ I'm happy to see that a 64 bit version is in progress. The installer didn't like my Windows 2003 x64, but once I extracted the files with 7-zip, I got a working 64 bit browser. I vote to warn, but allow installation on windows 2003 x64.
Note that "vote for this bug" means clicking on the "vote" link in the header. https://bugzilla.mozilla.org/page.cgi?id=etiquette.html
Thanks jj, I didn't know that. :) OK, I clicked (vote) near Importance and then added a tick in the check box. But I'm not sure if it worked, because there is neither a submit button, nor any kind of confirmation. And if I go back on this page, I still see one vote and the list doesn't include me. The interface for voting is confusing! Does it work in Chromium? ;)
After ticking the box, you must press the 'Change my votes' button on the bottom of the page.
I'm gonna dupe this with bug 809697 as it has a bit more details.