Waterfox is specially designed for x64. It's maybe a bit faster than ff... no rave rating from me on my system. Didn't check it for memory leaks.

It does nothing spectacularly, offers very little in the areas of speed and reliability over Firefox and is broken for about 2/3's of the plug-ins you might want to use with many problems in the streaming video category.

It is less buggy than Firefox (especially in it's current incarnation) but really incompatible or bugged in a lot of other areas.

That's my experience of using it in Win 7 since it came out.

I used it but went back to the Palemoon x64 version when v11 was released. I just did a clean OS install and I don't have any 32 bit browsers installed other than IE which I haven't even opened.

For all the plugins/addons I use, the only one that won't work with x64 is the Google Video/Voice/Talk/Chat.

At present, it seems to offer very little in the way of improvements and has many plug-ins which aren't compatible with it.