Also, Waterfox has had usable hardware acceleration before even mainstream *Firefox* (a major case where x64 provided a benefit that x32 lacked).
Waterfox has been usable (by anyone) almost from the get-go - I *still* have not seen an .EXE-based installer for x64 Chrome OR Opera.
Waterfox is not an official product which is supported by mozilla. And as for the installers... I am not sure if you have checked the Opera site. Others have posted the link here btw.