Waterfox is a version of Mozilla Firefox which has been designed especially for 64 bits systems.
Mozilla has not an official version, but Waterfox compete with Pale Moon.
Waterfoz navigator is totally compatible with all extension which has Firefox. However, you must have version for 64 bits for some plug-ins for example Flash, Java, Silverlight...
Waterfox and standard Firefox can coexist without problem, and they share the same user profile. The only difference that you will note an improvement in the performance of the navigator because you will have a 64 bits system, and when you will have a lot of tabs open you will notice the improvement.
No more to add because there are no major changes compared to original Firefox. In fact, for each updates of navigator of Mozilla, Waterfox also keeps to always offer reliably alternative to the traditional 32bits Firefox. So if you have just bought a 64 bits system... Why do you get its full potential on the internet? Download now and try this navigator in your 64 bits system.