After reading CNET’s review of Windows XP x64, it looks like this might be the case, if only temporarily. CNET notes that although Windows XP x64 defaults to the 64-bit version of IE, Microsoft’s does not yet have a 64-bit version of Windows Update for IE, so one has to run the 32-bit version of IE (both 32-bit and 64-bit IE are included) to get Windows Update functionality.
I’m not familiar with the internals of IE, but based on my understanding of the above, it looks like if someone was running the 64-bit version of Internet Explorer, a 32-bit BHO would probably not be able to execute, thereby thwarting the current crop of [ad|spy|mal]ware targeted at IE users.
Certainly as people begin to write malicious software targeted at 64-bit systems this “advantage” will disappear, but early adopters of Windows XP x64 might get a bit of a free ride, at least with respect to IE’s security.
Now that Windows XP x64 has been released, we should decide when we will begin supporting it by providing official 64-bit builds of Firefox/Thunderbird. Plugins will probably be an issue, but since extensions are not compiled, they should have no problems running. Work has started on this front and is being tracked in bug 237202. Third party Windows x64 Firefox builds are already available.