You're running a 32bit viewer here on 32bit Windows, so of course the 32bit viewer is going to use much less RAM. 32-bit programs can only address a maximum of 4GB of memory in a 64-bit OS and can only access 2GB of memory in a 32-bit OS, or 3GB if the program is built using LAA (large address aware).
Another thing to remember about comparing Firestorm 64bit & the LL viewer 64bit viewers is that the LL viewer still limits your texture memory to 512MB.
Firestorm allows texture memory up to 2GB (depending on how much memory your graphics card has available).
For reasons that are over my head (but @Ansariel Hiller can maybe explain in more detail), the higher you have your texture memory set to in the viewer, the more RAM that viewer will use also. The extra memory allowed for texture memory is not only reserved for the texture memory but is also added to the RAM use.
So when comparing FS 64bit to the LL viewer 64bit memory use, you need to limit your FS 64bit texture memory to 512MB or the comparison is meaningless.
You also need to make sure other settings that can affect memory use are the same.
For example: Draw distance, Object LOD factor, graphics quality settings/ALM/shadows etc, whether texture compression is enabled or disabled, whether OpenGL vertex buffer objects is enabled or disabled etc etc.
You also need to use the same avatar to test both viewers because inventory size will affect the amount of memory the viewer uses - each inventory item (where an item is a single asset or a folder) will take up about 7kb of memory.
Inventory size can make a HUGE difference to the amount of memory the viewer (any viewer) uses. For example, when I'm logged in with Whirly, the viewer will use in excess of an extra 1.5GB RAM then when I'm logged in on one of my alts with smaller inventory. That memory difference is solely because of Whirly having a big inventory.
Obviously you also need to be testing memory use in a location that's static with no avatars coming & going & nothing new being rezzed. A few high poly meshes, either worn or rezzed can alter memory use by a LOT.
For example I have some nasty high poly mesh items that when rezzed or worn will gobble an extra 2GB RAM & will outright crash a 32bit viewer with an out of memory crash.
Because of this you need to be wearing the exact same avatar attachments when comparing memory between viewers.
I don't want to be rude but there is a lot of terrible testing & conclusions being drawn on this thread.