Flash, Java, Silverlight, and so on all have 64-bit variants which work, but there's just no way to get the Google Talk and Hangouts plugin to work on those builds of Firefox.
Seeing the title gave me a bit of hope until I thought "No, of course, Google just bundles the plugin anyways... not like they'll publish a download link for a 64-bit build". I wouldn't even care if you had to click through several "Other system / Select custom download" links, if only it were available at all.
I feel like we have a chicken-egg problem. Firefox isn't offering 64-bit prominently because plugins have terrible support for it, and plugins don't support it because it doesn't exist yet... But, unlike Firefox, plugins can offer 64-bit compatible versions without angering a large number of users. Really, the ball should be in their court, Google included, to provide the options.