1. Close Firefox
2. Open the Applications folder in Finder
3. Right click or control-click the Firefox.app icon
4. Select "Get Info"
5. Select or Deselect "Open in 32-bit mode"
6. Close the "Firefox Info" window
7. Restart Firefox

I found this line added to the above instructions which kind of help answer:

If you want to revert to running Firefox 4 in 64-bit mode, then you can use those same instructions to uncheck the 32-bit box.

SO I guess we are to assume that Firefox is automatically running in 64 when it's restarted UNLESS that box is checked.

The thing I don't understand is since I also reverted to 32 bit after Hotmail kept nagging me too, and then reopened Firefox (supposedly in 64 bit), I no longer get the nagging prompt to change BACK to 32.

Who knows what's going on?

You run Firefox in 32 bit mode as long as "Open in 32-bit mode" has a check mark.
You need to remove the check mark if you want to start in 64 bit mode.