14 DAY TRIAL // Although Mozilla’s been making 64-bit builds for Firefox for quite a long time, none of them received the official stamp approval to be released. But if you want to try some Firefox code optimized for 64-bit architecture, you can try Waterfox.
The developer of the application actually took the Firefox source code and compiled it to run on x64 Windows. Optimizations for 64-bit platform are included, so you should get better browsing speed.
According to the developer, Waterfox fares better in benchmarks compared to the regular Firefox. As far as add-ons are concerned, they should all work just fine. It also supports the most used plug-ins (Flash, Java and Silverlight).
Since Waterfox spawned from Firefox code and the developer compiled and optimized it to run smoothly on 64-bit platforms, any bugs you may encounter have to be fixed by the author of the code, namely Mozilla.