Waterfox is basically a 64-Bit version of Firefox. The Firefox source code is taken and compiled to run specifically for 64-Bit Windows computers. To make Waterfox stand out a bit more, it’s compiled with optimizations so that it will run more efficiently and faster than just compiling Firefox as a 64-Bit program. For some people with older systems, the 64-Bit version loads quicker and is much more responsive than the 32-Bit build.
In benchmarks, the 64-Bit variant of Firefox out-performs the 32-Bit variant. Also because this variant is being built specifically for Windows, there might be further performance increases. Waterfox was compiled with SSE, SSE2, x64 favoring and the following optimisation flags: /Og /Oi /Ot /Oy /Ob2 /Gs /GF /Gy
Add-Ons work on any version of Firefox, on any platform.