The story of 64-bit Firefox on Windows is a long and complex one, and it is just about to end with Mozilla moving from offering 32-bit Firefox as the default download option to offering 64-bit Firefox on Windows by default.
Mozilla Firefox is offered as a 32-bit and 64-bit version on Windows, with 32-bit still the default right now when it comes to downloads.
While the 32-bit version works well, and it is the only option for systems without a 64-bit processor, it is the 64-bit version that users should consider using if their device is equipped with a 64-bit cpu.
The reason is simple: more RAM becomes available per process that Firefox uses, and 64-bit applications benefit from security features that 32-bit applications don't.