64-bit Firefox for Windows is mostly identical to 32-bit Firefox for Windows, except that very few plug-ins will work with 64-bit Firefox. This is by design: Mozilla is in the process of dropping Firefox’s support for NPAPI plug-ins. NPAPI support is being dropped due to (ostensible) stability and security concerns. Amusingly (or ironically), though, 64-bit Firefox does still support one plug-in: Flash. Sites that use other NPAPI plug-ins, such as Silverlight or Java, are being told by Mozilla to "accelerate their transition to Web technologies."