If you want to try some Firefox code optimized for 64-bit architecture, you can try Waterfox.

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.

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.

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).

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.