JJ,

It's not possible to prevent a resource of an IFRAME from being
loaded using GM script.

I'm not trying to prevent it from loading, I'm removing the iframe itself,
as I have no need for its contents.

.... but which you could refer to as a kind of prevention too I guess. :-)

The problem is that GreaseMonkeys scripted changes are applied only after
FireFox has already decided to download the iframes contents.

And thats a bit different from how, for instance, an IMG element is handled:
When you remove it the resource it pointed to will not be downloaded (if I
may believe RequestPolicy).

Regards,
Rudy Wieser